docs: def get_source_files(target, build_context) -> list:
      """Return list of source files for `target`."""
      all_sources = list(target.props.sources)
      for proto_dep_name in target.props.protos:
          proto_dep = build_context.targets[proto_dep_name]
          all_sources.extend(proto_dep.artifacts.get(AT.gen_cc).keys())
      return all_sources