This also fixes maven to package this correctly as google/protobuf/java_features.proto (same dir as WKT/descriptor.proto) instead of com/google/protobuf/java_features.proto.
Fixes https://github.com/protocolbuffers/protobuf/issues/16155
PiperOrigin-RevId: 619015714
protobuf/java will become a parent pom that will contain two modules:
core - contains all of the code for the protobuf-java artifact
util - contains all of the code for the protobuf-java-util artifact
Also cleaned up various Maven warnings.