This big breaking change will force users of the Protobuf rules to use the Starlark rules from the @rules_proto repository. The rules there are currently only wrappers around native rules, but they will be rewritten in the future.

Design doc: https://docs.google.com/document/d/1Wai_okIK_NnhNJZai5nkTUvZVHdV5pp1YwNLFtWwpC4/edit?ts=5d2c8077