Include a directory of unpackaged files in assemble_targz

[adammitchelldev]

There is no simple way to include a directory of files that has not been packaged using the assemble_targz rule. In this instance, I would like to include a filegroup into the targz package. The current additional_files map prevents me from adding multiple files in their original directory structure, which is quite limiting.

I would suggest something similar to the data attribute in java_library, which uses standard Bazel labels, or adding the ability to use targets in the same way in the additional_files map.