Expose source files from an rb_gem as rb_library or java jars

Would like to be able to directly reference a specific source file included in an `rb_gem` installed in a bundle repo.

For example to use the [warbler](https://github.com/jruby/warbler) jar in a java based target elsewhere.

```
java_import(
name = "WarblerJar",
jars = [".gem/gems/warbler-2.0.4/lib/warbler_jar.jar"],
)
```

Right now you must reference a file in the file system outside of bazel and know its specific path. I'm guessing we'd need bundle install to run to unpack the .gem archive, and then generate rb_library targets and java targets based on the outputted file extensions after.