This commit is intended to create the possibility to plug in into
the compiler an alternative classpath representation which would be
possibly more efficient, use less memory etc. Such an implementation
- at least at the beginning - should exist next to the currently
existing one and be possible to turn on using a flag.

Several places in the compiler have a direct dependency on the
classpath implementation. Examples include backend's icode generator
and reader, SymbolLoaders, ClassfileParser. After closer inspection,
one realizes that all those places depend only on a very small subset
of classpath logic: they need to lookup classes from classpath. Hence
there's introduced ClassFileLookup trait that encapsulates that
functionality. The ClassPath extends that trait and an alternative
one also must do it.

There's also added ClassRepresentation - the base trait for ClassRep
(the inner class of ClassPath). Thanks to that the compiler uses
a type which is not directly related to the particular classpath
representation as it was doing until now.

The method asClasspathString is now deprecated. Moreover it's moved
to ClassFileLookup in the case someone was using it in some project
(an alternative classpath also will support it - just in the case).
All its usages existing in Scala sources are changed to
asClassPathString method. The only difference is the name.

Some operations on files or their names are moved from ClassPath to
the newly created FileUtils dedicated to classpath. It will be
possible to reuse them when implementing an alternative classpath
representation. Moreover such allocation-free extension methods like
the one added in this commit will improve the readability.

This commit introduces the base trait for flat classpath - an
alternative classpath representation. In accordance with the idea
and the experimental implementation of @gkossakowski, this
representation will try to make the best use of the specificity of
a given file type instead of using AbstractFile everywhere. It's
possible as .NET backend is no longer supported and we can focus on
Java-specific types of files.

FlatClassPath extends ClassFileLookup which provides the common
interface used also by existing ClassPath.

The new implementation is called flat because it's possible to query
the whole classpath using just single instance.
In the case of the old (recursive) representation there's the
structure of nested classpath objects, where each such an object can
return only entries from one level of hierarchy but it returns also
another classpath objects for nested levels included in it.

That's why there's added dedicated PackageLoaderUsingFlatClassPath in
SymbolLoaders - approaches are different so also the way of loading
packages has to be different. The new package loader is currently
unused.

There's added also PackageNameUtils which will provide common methods
used by classpath implementations for various file types.

There's added AggregateFlatClassPath - an equivalent of
MergedClassPath from the old implementation. It is supposed to group
classpath instances handling different files being directories, zips
or jars.

Unlike in the case of the old (recursive) implementation, there won't
be a deep, nested hierarchy of classpath instances - just one root
(aggregate) and a flat structure of its children.

AggregateFlatClassPath ensures the distinction of classpath entries
and merges corresponding entries for class and source files into one
entry. This is required as SymbolLoaders class makes use of this kind
of ClassRepresentation.

There are also added unit tests which check whether
AggregateFlatClassPath obtains correct entries from classpath
instances specified in a constructor and whether it preserves the
ordering in the case of repeated entries. There's added a test type
of flat classpath using VirtualFiles so it's easy to check the real
behaviour.

There's added the flat classpath implementation for directories using
java.util.File directly. Since we work with a real directory - not
the AbstractFile - we don't need to iterate all entries of a file to
get inner entries of some package. We can just find an adequate
directory for a package.

There are added implementations for a class- and a sourcepath. Both
extend DirectoryFileLookup which provides common logic.

This commit adds an implementation of flat classpath which can handle
both jar and vanilla zip files. In fact there are two versions - for
a class- and a sourcepath. Both extend ZipArchiveFileLookup which
provides common logic.

They use FileZipArchive. @gkossakowski made a comparison of different
ways of handling zips and jars (e.g. using javac's ZipFileIndex). He
stated that general efficiency of FileZipArchive, taking into account
various parameters, is the best.

FileZipArchive is slightly changed. From now it allows to find the
entry for directory in all directory entries without iterating all
entries regardless of a type. Thanks to that we can simply find
a directory for a package - like in the case of DirectoryFileLookup.

There's also added possibility to cache classpath representation of
classpath elements from jar and zip files across compiler instances.
The cache is just a map AbstractFile -> FlatClassPath. It should
reduce the number of created classpath and file instances e.g. in the
case of many ScalaPresentationCompilers in Scala IDE.

To prevent the possibility to avoid a cache, caches are created as
a part of factories responsible for the creation of these types of
the flat classpath.

There's added the flat classpath type using ManifestResources,
closely related to the support for JSR-223 (Scripting for the Java
Platform). It uses classes listed in the manifest file placed in the
JAR. It's related to jar files so it's created using
ZipAndJarFlatClassPathFactory and is cached.

In general currently it's not possible to use it in Scala out of the
box (without using additional tools such as jarlister) as this
support is postponed. The old classpath has been properly prepared in
the PR created by @rjolly scala#2238
so the new one also got this feature.

ManifestResources is a ZipArchive without a real underlying file
placed on a disk and in addition implementing some methods declared
in AbstractFile as unsupported operations. Therefore the
implementation has to use the iterator. I wanted to have the similar
behaviour as in the case of directories and zip/jar files - be able
to get a directory entry for a package without iterating all entries.
This is achieved by iterating all entries only once and caching
packages.

This flat classpath type was the last needed one.

The part of the functionality of a ClassPathContext has been moved
to the base trait ClassPathFactory so it can be reused by the newly
created FlatClassPathFactory. This new implementation works in
similar manner as the ClassPathContext with this difference that it
just creates instances of flat classpath.

This change doesn't modify the behaviour of the compiler as the
interface and the way ClassPathContext works didn't change. Moreover
FlatClassPathFactory is currently unused.

This commit adds dedicated FlatClassPathResolver loading classpath
entries as FlatClassPath.

Most of the common logic from PathResolver for the old classpath has
been moved to the base, separate class which isn't dependent on
a particular classpath representation. Thanks to that it was possible
to reuse it when creating an adequate path resolver for the flat
classpath representation.

This change doesn't modify the way the compiler works. It also
doesn't change nothing from the perspective of someone who already
uses PathResolver in some project or even extends it - at least as
long as he/she doesn't need to use flat classpath.

There are also added JUnit tests inter alia comparing entries created
using the old and the new classpath representations (whether the flat
one created using the new path resolver returns the same entries as
the recursive one).

The structure of scalap's Main has been refactored.

EmptyClasspath is deleted. It looks that it was unused since this
commit: scala@e594fe5

Also classpath logging is changed and now uses asClassPathString
method. It was needed to modify one test because of that but it
won't depend on a particular representation.

There aren't changes in the way scalap works.

This commit integrates with the compiler the whole flat classpath
representation build next to the recursive one as an alternative.

From now flat classpath really works and can be turned on. There's
added flag -YclasspathImpl with two options: recursive (the default
one) and flat.

It was needed to make the dynamic dispatch to the particular
classpath representation according to the chosen type of a classpath
representation.

There's added PathResolverFactory which is used instead of a concrete
implementation of a path resolver. It turned out that only a small
subset of path resolvers methods is used outside this class in Scala
sources. Therefore, PathResolverFactory returns an instance of a base
interface PathResolverResult providing only these used methods.

PathResolverFactory in combination with matches in some other places
ensures that in all places using classpath we create/get the proper
representation.

Also the classPath method in Global is modified to use the dynamic
dispatch. This is very important change as a return type changed to
the base ClassFileLookup providing subset of old ClassPath public
methods. It can be problematic if someone was using in his project
the explicit ClassPath type or public methods which are not provided
via ClassFileLookup. I tested flat classpath with sbt and Scala IDE
and there were no problems. Also was looking at sources of some other
projects like e.g. Scala plugin for IntelliJ and there shouldn't be
problems, I think, but it would be better to check these changes
using the community build.

Scalap's Main.scala is changed to be able to use both implementations
and also to use flags related to the classpath implementation.

The classpath invalidation is modified to work properly with the old
(recursive) classpath representation after changes made in a Global.
In the case of the attempt to use the invalidation for the flat cp it
just throws exception with a message that the flat one currently
doesn't support the invalidation. And also that's why the partest's
test for the invalidation has been changed to use (always) the old
implementation. There's added an adequate comment with TODO to this
file.

There's added partest test generating various dependencies
(directories, zips and jars with sources and class files) and testing
whether the compilation and further running an application works
correctly, when there are these various types of entries specified as
-classpath and -sourcepath. It should be a good approximation of real
use cases.

There's added -YdisableFlatCpCaching option to ScalaSettings which
allows user to disable caching of flat representation of classpath
elements.

This commit contains some minor changes made by the way when
implementing flat classpath.

Sample JUnit test that shows that all pieces of JUnit infrastructure
work correctly now uses assert method form JUnit as it should do from
the beginning.

I removed commented out lines which were obvious to me. In the case
of less obvious commented out lines I added TODOs as someone should
look at such places some day and clean them up.

I removed also some unnecessary semicolons and unused imports.

Many string concatenations using + have been changed to string
interpolation.

There's removed unused, private walkIterator method from ZipArchive.
It seems that it was unused since this commit:
scala@9d4994b
However, I had to add an exception for the compatibility checker
because it was complaining about this change.

I made some trivial corrections/optimisations like use 'findClassFile'
method instead of 'findClass' in combination with 'binary' to find
the class file.

The goal of these changes is to add possibility to:
- compare an efficiency and a content of both cp implementations
(ClassPathImplComparator)
- examine the memory consumption by creating a lot of globals using
a specified classpath (ClassPathMemoryConsumptionTester) - it can be
considered as e.g. some approximation of ScalaPresentationCompilers
in Scala IDE when working with many projects

ClassPathMemoryConsumptionTester is placed in main (I mean not test)
sources so thanks to that it has properly, out of the box configured
boot classpath etc. and it's easy to use it, e.g.:

scala scala.tools.nsc.ClassPathMemoryConsumptionTester
-YclasspathImpl:<implementation_to_test> -cp <some_cp>
-sourcepath <some_sp> -requiredInstances 50 SomeFileToCompile.scala

At the end it waits for the "exit" command so there can be used some
profiler like JProfiler to look how the given implementation behaves.

Also flat classpath implementation is set as a default one to test it
on Jenkins. This particular change must be reverted when all tests
will pass because for now it's not desirable to make it permanently
the default representation.

This commit addresses code review comments.

The flat classpath is no longer the default classpath representation.
It was the default one just for the test purposes. For now it's not
desirable to make it permanently the default representation.

ZipAndJarFileLookupFactory is marked as sealed - it should help to
limit the ways of creating flat classpath instances for zips and jars.