cmd/protoc-gen-go: make internal dependencies more patch-friendly
cmd/protoc-gen-go: make internal dependencies more patch-friendly

Declare the internal protobuf dependencies as variables of interface
type so that they can be more easily replaced by custom implementations.

Change-Id: I7fff885fd79ee0117c1c62654b2fd4b1877708da
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/186659
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go