Consider restructuring python targets to be 1-1-1

[jsirois]

See #385 for the java side of this. In java, fine grained and deep package structures are more of-the-norm. In python, less so; so this issue is phrased as a "consider". The idea though is to eliminate most or all exclude lists in sources globs in the python codebase. This would require re-structuring packages and thus risk breaking clients imports.