Make plugin use betterproto generated classes internally

This means the betterproto plugin no longer needs to depend durectly on
protobuf.

This requires a small runtime hack to monkey patch some google types to
get around the fact that the compiler uses proto2, but betterproto
expects proto3.

Also:
- regenerate google.protobuf package
- fix a regex bug in the logic for determining whether to use a google
  wrapper type.
- fix a bug causing comments to get mixed up when multiple proto files
  generate code into a single python module