Support new protoc-gen-go which doesn't support import_path

[kzys]

protoc-gen-go in github.com/protocolbuffers/protobuf-go is the new version of protoc-gen-go. However, the new version doesn't support import_path which protobuild uses.

We need to figure out the way to use the new version to keep up with the protobuf ecosystem.

[kzys]

Resolved by #48.