SI-8502 Improve resiliance to absent packages

[retronym]

When unpickling a class, we create stub symbols for references
to classes absent from the current classpath. If these references
only appear in method signatures that aren't called, we can
proceed with compilation. This is in line with javac.

We're getting better at this, but there are still some gaps.
This bug is about the behaviour when a package is completely
missing, rather than just a single class within that package.

To make this work we have to add two special cases to the unpickler:

• When unpickling a ThisType, convert a StubTermSymbol into
  a StubTypeSymbol. We hit this when unpickling
  ThisType(missingPackage).
• When unpickling a reference to <owner>.name where <owner>
  is a stub symbol, don't call info on that owner, but rather
  allow the enclosing code in readSymbol fall through to
  create a stub for the member.

The test case was distilled from an a problem that a Spray user
encountered when Akka was missing from the classpath.

Review by @gkossakowski

[retronym]

Here is how to reproduce the failure from the command line:

% tail sandbox/{b,c}.scala; scalac-hash v2.11.4 sandbox/{b,c}.scala && rm vanishing/Vanishing.class && rmdir vanishing && scalac-hash v2.11.4 sandbox/c.scala
==> sandbox/b.scala <==
object U {
  def foo(log: vanishing.Vanishing) = ()
}

package vanishing {
  class Vanishing
}

==> sandbox/c.scala <==
trait Test {
  U
}
error: scala.reflect.internal.Types$TypeError: missing or invalid dependency detected while loading class file 'U.class'.
Could not access term vanishing in package <root>,
because it (or its dependencies) are missing. Check your build definition for
missing or conflicting dependencies. (Re-run with `-Ylog-classpath` to see the problematic classpath.)
A full rebuild may help if 'U.class' was compiled against an incompatible version of <root>.
    at scala.tools.nsc.typechecker.Contexts$ThrowingReporter.handleError(Contexts.scala:1387)
    at scala.tools.nsc.typechecker.Contexts$ContextReporter.info0(Contexts.scala:1283)
    at scala.tools.nsc.typechecker.Contexts$ContextReporter.info0(Contexts.scala:1237)
    at scala.reflect.internal.Reporter.error(Reporting.scala:82)