protobuffers know what files they will generate for zips/jars.

Preliminary support for compressed archives as sources. Currently .zips and .jars work for protobuffers. Code is fairly extensible, with generic functionality in zipped_sources such that not much code is needed in protobuf_gen (or other targets in the future).
I'm not sure why this line is all the way up here when 'sources' isn't referenced until line 116