mocking.mock_data broken by https://github.com/tensorflow/datasets/commit/e8d8966e888667091875fd885ab0803a6dc5a383

/!\ PLEASE INCLUDE THE FULL STACKTRACE AND CODE SNIPPET

Short description
https://github.com/tensorflow/datasets/commit/e8d8966e888667091875fd885ab0803a6dc5a383 broken mock_data().

Environment information

• Operating System: Linux

• Python version: 3.7.7

• tensorflow-datasets/tfds-nightly version: 4.5.2.dev202203220044

• Does the issue still exists with the last tfds-nightly package (pip install --upgrade tfds-nightly) ?

Reproduction instructions

  with mock_data(num_examples=40):
    builder = tfds.builder("imagenet2012", "...")
    ... 

Link to logs

input_tfds.py:74: in _build_dataset
    builder = tfds.builder(cfg.dataset_name, data_dir=cfg.data_dir)
/miniconda/envs/py377/lib/python3.7/site-packages/tensorflow_datasets/core/load.py:149: in builder
    community.community_register.has_namespace(name.namespace)):
/miniconda/envs/py377/lib/python3.7/site-packages/tensorflow_datasets/core/community/registry.py:124: in has_namespace
    return namespace in self.registers_per_namespace
/miniconda/envs/py377/lib/python3.7/site-packages/tensorflow_datasets/core/community/registry.py:121: in registers_per_namespace
    return self.namespace_config.registers_per_namespace()
/miniconda/envs/py377/lib/python3.7/site-packages/tensorflow_datasets/core/community/registry.py:93: in registers_per_namespace
    config = toml.loads(self.config_path.read_text())
/miniconda/envs/py377/lib/python3.7/site-packages/etils/epath/abstract_path.py:141: in read_text
    return f.read()
/miniconda/envs/py377/lib/python3.7/site-packages/tensorflow/python/lib/io/file_io.py:114: in read
    self._preread_check()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
 self = <tensorflow.python.platform.gfile.GFile object at 0x7f4e18542450>
     def _preread_check(self):
      if not self._read_buf:
        if not self._read_check_passed:
          raise errors.PermissionDeniedError(None, None,
                                             "File isn't open for reading")
        self._read_buf = _pywrap_file_io.BufferedInputStream(
>           compat.path_to_str(self.__name), 1024 * 512)
E       tensorflow.python.framework.errors_impl.NotFoundError: /miniconda/envs/py377/lib/python3.7/site-packages/tensorflow_datasets/community-datasets.toml; No such file or directory
 /miniconda/envs/py377/lib/python3.7/site-packages/tensorflow/python/lib/io/file_io.py:77: NotFoundError

Expected behavior
tfds does not try to read community-datasets.toml.

Additional context
Add any other context about the problem here.

Reproduce the failure with mocking.mock_data and tfds.builder using the supplied Python snippet, then inspect the community registry path reached through has_namespace and registers_per_namespace. Compare that behavior with commit e8d8966e888667091875fd885ab0803a6dc5a383; done means mock_data no longer attempts to read the missing community-datasets.toml file.