support for java_plugins within scala_library rules

The built in [java_library](https://docs.bazel.build/versions/master/be/java.html#java_library) rule has support for [java_plugins](https://docs.bazel.build/versions/master/be/java.html#java_plugin) by way of the `plugins` attribute. scala_library also has a `plugins` attribute but I believe its only intended for scalac plugins, as one would expect.

Since the scala rules support compiling both java and scala sources, has anyone here had experience with integrating java a annotation processor with scala rules. If not, does anyone here have pointers on where to look?

Start with the linked java_library and java_plugin documentation, then compare it with the scala_library plugins attribute described in the issue. No source files, tests, or settled expected behavior are named; progress requires defining whether Java annotation processors should be supported and how that support would be verified.

Written by the indexing model from the issue text.