Should `scala_import` return jar output file provider (in addition to java_common provider)?

During work on this [PR](https://github.com/bazelbuild/rules_scala/pull/473) I've noticed that location expansion of jars exposed by `scala_import` fails:
For [example](https://github.com/bazelbuild/rules_scala/blob/aaa6c7d4db4f231a541f20a60760420f4bdd11e8/third_party/plugin/src/test/BUILD#L19)
`$(location @com_google_guava_guava_21_0//jar)` fails with `label '@com_google_guava_guava_21_0//jar:jar' in $(location) expression expands to no files.` when the jar target is of type `rules_scala`

So it seems that `scala_import` is missing a feature for exposing jar output files.

@lberki are bazel rules idiomatically expected to output files?