Include java_sources in targets.
the final target set returned by context.targets() was not considering targets that weren't specified in the "dependencies" of thescala_library. In this case, a target was specified using "scala_library:java_sources" which was being dropped. This in turn was causing idea not to load the project with the java sources it should have had.

Modified ScalaLibrary to return its own implementation of closure()
which includes java_sources.

Testing Done:
1) $./pants goal idea src/scala/com/pants/testproject/javasources::
2) https://travis-ci.org/pantsbuild/pants/builds/30893765

Bugs closed: 390

Reviewed at https://rbcommons.com/s/twitter/r/733/
Merge branch 'master' of https://github.com/pantsbuild/pants