Having the grpc bindings in the pyln-testing package was always a bit
strange, however it came with additional issues. Due to the way that
protos are handled by protobuf, any name clash, independently of where
we import the protos, would cause an import error. This usually
happens in diamond-pattern dependencies, and so pull out the generated
files into their own package that everyone else can rely on.
Changelog-None
Christian Decker