gwt_binary() uses Bazel feature to retrieve the sources from the
dependency libraries. For the standalone API this feature cannot be
used as the only dependency is GWT UI plugin API. Expose the sources
in GWT UI plugin API as well.
TEST PLAN:
Apply this change and create the plugin API:
$ ./tools/maven/api.sh install
Apply reviewers plugin Bazel build implementation change: [1] and
verify, that the reviewers plugin can be built with the GWT UI plugin
API created with this change.
[1] https://gerrit-review.googlesource.com/91830
Change-Id: I2b065a8eccbc33020d461834704d684871f463d7
8c9a9ea486