This reverts commit 79759fea, reversing
changes made to dc36f4df.

While not strictly required by the protocol,
it will make testing easier.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Some features relating to proto2 extension querying are missing due to
missing upstream features required to support them (see
https://github.com/google/protobuf/issues/2248).