Fix java_sources missing dep detection.

This seems to just be an oversight in the original implementation.
We noticed incorrect missing dependency errors (we have it set to
error rather than warn) when we started using java_sources in our codegen.
This resolved our issue internally.

The general idea is that a ScalaLibrary should be claiming its
java_sources sources for the purpose of mapping generated classfiles
back to the sources zinc used to generate them.

Testing Done:
CI is green: https://travis-ci.org/pantsbuild/pants/builds/41043159

Reviewed at https://rbcommons.com/s/twitter/r/1336/
Sync of https://github.com/pantsbuild/pants/commit/04904f750c993971303d9b64720bcaf620eea00a