Expose source files from an rb_gem as rb_library or java jars

[noahkawasakigoogle]

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 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.

[p0deje]

You are right, we'd need to install the gem and then expose all jars by globbing *.jar inside gem installation dir. Happy to merge PR adding this support!