Fixup a hidden jar <-> publish dependency cycle.

This simplifies JarCreate letting it just create classfile jars and
moves the responsibility of creating source and doc jars to the
only user - JarPublish.

Testing Done:
./build-support/bin/ci.sh

travis-ci went green here: https://travis-ci.org/pantsbuild/pants/builds/29627541

Bugs closed: 335

Reviewed at https://rbcommons.com/s/twitter/r/660/