Add support for an explicit srcjar for scala_import

It would be nice to have `scala_import` be on par with [`java_import`](https://docs.bazel.build/versions/master/be/java.html#java_import) in this regard, especially since `scala_import_external` suggests it should be supported.

I could probably take on fixing this if somebody could mentor me regarding a preferred solution.

Start by comparing the existing scala_import behavior with java_import's explicit srcjar support and the scala_import_external rule, which the issue identifies as relevant entry points. Trace how srcjars are represented and determine what tests cover these rules; done means scala_import accepts and uses an explicit srcjar consistently with the requested behavior.

Written by the indexing model from the issue text.