Migrate v2 from github.com/golang/protobuf to google.golang.org/protobuf

[pdecks]

Notable changes between google.golang.org/protobuf and github.com/golang/protobuf:

1. import_prefix is deprecated and go_package should be used instead
2. the signature for proto.GetExtension(...,...) changed from (interface{}, error) to interface{}
3. proto.GetExtension(pb, ext) now panics for nil, nil

[pdecks]

I'm trying to figure out why the other protobuf library is being installed by make testdata-v2 🤔 (see check output)

[pdecks]

Made some progress, on cb8c28f debugging import_prefix error in make testdata command:

cd testdata/packages && \
set -e; for subdir in `find . -mindepth 1 -maxdepth 1 -type d | grep -v targets`; do \
	cd $subdir; \
	params=`cat params`; \
	protoc -I . -I .. \
		--plugin=protoc-gen-debug=../../../../../../bin/protoc-gen-debug \
		--debug_out=".:." \
		--go_out="paths=source_relative,$params:." \
		`find . -name "*.proto"`; \
	cd -; \
done
/home/runner/work/protoc-gen-star/protoc-gen-star/src/github.com/lyft/protoc-gen-star/v2/lang/go/testdata/packages
/home/runner/work/protoc-gen-star/protoc-gen-star/src/github.com/lyft/protoc-gen-star/v2/lang/go/testdata/packages
protoc-gen-go: no such flag -import_prefix
--go_out: protoc-gen-go: Plugin failed with status code 1.
make[1]: *** [Makefile:16: testdata-packages] Error 1
make[1]: Leaving directory '/home/runner/work/protoc-gen-star/protoc-gen-star/src/github.com/lyft/protoc-gen-star/v2/lang/go'