Records the module that defines a specific flag.

    Args:
      module_id: int, the ID of the Python module.
      flag: Flag, the Flag instance that is key to the module.

def register_flag_by_module_id(self, module_id, flag):
    """Records the module that defines a specific flag.

    Args:
      module_id: int, the ID of the Python module.
      flag: Flag, the Flag instance that is key to the module.
    """
    flags_by_module_id = self.flags_by_module_id_dict()
    flags_by_module_id.setdefault(module_id, []).append(flag)