Function proto_path_to_stem 

pub fn proto_path_to_stem(proto_path: &str) -> String

Convert a .proto file path to its content-file stem.

e.g., "google/protobuf/timestamp.proto" → "google.protobuf.timestamp". The five content files append "", ".__view", ".__oneof", ".__view_oneof", ".__ext" plus ".rs".