reduce the copies of the core google.protobuf protos

[devoncarew]

We have another copy of the google.protobuf protos here: protoc_plugin/test/protos/google/protobuf/. We should de-dupe them with the copies in protoc_plugin/protos.

[osa1]

They're not the same protos.

• protoc_plugin/protos: protos needed to talk to protoc.
• protoc_plugin/test/protos/google/protobuf: protos for testing.