This opens the door for other serializers (such as e.g. FlatBuffers)
to share this code generator when using GRPC.

This reverts commit 5b304753.

The new spec uses existing EchoDuplicate service in
grpc.testing.duplicate, which refers grpc.testing.Empty,
and verifies if the plugin generates this correctly as
Grpc::Testing::Empty.

- Use full_name() instead of name(), otherwise package names
  are omitted.
- Fix a minor bug on RubyTypeOf(), to replace dotted protobuf
  package name by ruby's module names correctly.