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

Preliminary support for compressed archives as sources. Currently .zips, .jars, tars, tar.gzs, and tar.bz2s 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).
Trying to find the 'package' the .zip is in, because of where protoc expects files to be arranged, for use in building the directory structure of the extracted temporary files.