Records the module that defines a specific flag.

    We keep track of which flag is defined by which module so that we
    can later sort the flags by module.

    Args:
      module_name: str, the name of a Python module.
      flag: Flag, the Flag instance that is key to the module.

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

    We keep track of which flag is defined by which module so that we
    can later sort the flags by module.

    Args:
      module_name: str, the name of a Python module.
      flag: Flag, the Flag instance that is key to the module.
    """
    flags_by_module = self.flags_by_module_dict()
    flags_by_module.setdefault(module_name, []).append(flag)