docstr: When entering the context, the values of ``packages``, ``py_modules`` and
  ``package_dir`` that are missing in ``dist`` are copied from ``setuptools_cfg``.
code: def __enter__(self):
      """When entering the context, the values of ``packages``, ``py_modules`` and
      ``package_dir`` that are missing in ``dist`` are copied from ``setuptools_cfg``.
      """
      dist, cfg = self._dist, self._setuptools_cfg
      package_dir: Dict[str, str] = cfg.setdefault("package-dir", {})
      package_dir.update(dist.package_dir or {})
      dist.package_dir = package_dir  # needs to be the same object
  
      dist.set_defaults._ignore_ext_modules()  # pyproject.toml-specific behaviour
  
      # Set `name`, `py_modules` and `packages` in dist to short-circuit
      # auto-discovery, but avoid overwriting empty lists purposefully set by users.
      if dist.metadata.name is None:
          dist.metadata.name = self._project_cfg.get("name")
      if dist.py_modules is None:
          dist.py_modules = cfg.get("py-modules")
      if dist.packages is None:
          dist.packages = cfg.get("packages")
  
      return super().__enter__()
  
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 2 for calling the _EnsurePackagesDiscovered obj's underlying member method code with example usage: obj.__enter__() and returns: super