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 have it this way because it actually needed to be more deterministic, because protoc requires a base directory for all sources it compiles (because it's bad at distinguishing absolute from relative paths).