Use PackageWarning system for multiple file overwrite errors

[jcollins-g]

Fixes #2110.

This removes the assert and replaces it with Dartdoc's warning system.

By defaulting to error type, this will show an error for the webdriver package, but Flutter already uses a flag to disable all errors in packages it depends on (which will also silence this error for them).

So after this gets published, webdriver will get a descriptive error message saying what is wrong, and everyone else can go about their business.

Example output:

Initialized dartdoc with 39 libraries in 14.8 seconds
documenting test_package_duplicate
Generating docs for library aDuplicate from package:test_package_duplicate/second.dart...
  warning: aDuplicate has no library level documentation comments, from aDuplicate: (file:///Users/jcollins/dart/dartdoc/testing/test_package_duplicate/lib/second.dart:1:9)
Generating docs for library aDuplicate from package:test_package_duplicate/first.dart...
  warning: aDuplicate has no library level documentation comments, from aDuplicate: (file:///Users/jcollins/dart/dartdoc/testing/test_package_duplicate/lib/first.dart:1:9)
  error: failed to write file at: aDuplicate/aDuplicate-library.html
    for symbol aDuplicate: (file:///Users/jcollins/dart/dartdoc/testing/test_package_duplicate/lib/first.dart:1:9)
    conflicting with file already generated by aDuplicate: (file:///Users/jcollins/dart/dartdoc/testing/test_package_duplicate/lib/second.dart:1:9)
            Dartdoc generates a path and filename to write to for each symbol.
            aDuplicate conflicts with another symbol in the generated path, and
            therefore can not be written out.  Changing the name, library name, or
            class name (if appropriate) of one of the conflicting items can resolve
            the conflict.   Alternatively, use the @nodoc tag in one symbol's
            documentation comments to hide it.
Validating docs...
found 2 warnings and 1 error
Documented 2 public libraries in 0.2 seconds

dartdoc failed: dartdoc encountered 1 errors while processing..
jcollins-macbookpro2:test_package_duplicate jcollins$

[coveralls]

Coverage decreased (-0.009%) to 93.93% when pulling 09a4e8b on assert-failure-multiple-files into 88ba99b on master.