docstr: When exiting the context, if values of ``packages``, ``py_modules`` and
  ``package_dir`` are missing in ``setuptools_cfg``, copy from ``dist``.
code: def __exit__(self, exc_type, exc_value, traceback):
      """When exiting the context, if values of ``packages``, ``py_modules`` and
      ``package_dir`` are missing in ``setuptools_cfg``, copy from ``dist``.
      """
      # If anything was discovered set them back, so they count in the final config.
      self._setuptools_cfg.setdefault("packages", self._dist.packages)
      self._setuptools_cfg.setdefault(
          "py-modules", self._dist.py_modules
      )
      return super().__exit__(exc_type, exc_value, traceback)
  
imports: import logging
  import os
  from contextlib import contextmanager
  from functools import partial
  from typing import TYPE_CHECKING, Callable, Dict, Mapping, Optional, Set, Union
  from ..errors import FileError, InvalidConfigError
  from ..warnings import SetuptoolsWarning
  from .None import expand
  from ._apply_pyprojecttoml import _PREVIOUSLY_DEFINED, _MissingDynamic
  from ._apply_pyprojecttoml import apply
label_desc: Use image node_id 3 for calling the _EnsurePackagesDiscovered obj's underlying member method code with example usage: obj.__exit__(exc_type, exc_value, traceback) and returns: super