Migrate to using only java_import

[johnynek]

currently we do a repo rule that downloads the file, then use java_import to wrap that, then use java_library to build the dependency graph.

This is because in the past, java_import didn't allow exports or runtime_deps so we couldn't set up the dependency graph.

That's no longer the case, so we could just directly use java_import and scala_import and reduce the number of targets and also have a side benefit of avoiding producing the empty jars that are produced by java_library.

[johnynek]

scala_import seems to be doing the right thing, btw. So, this is only for java libraries.