code: def _find_already_built_wheel(metadata_directory):
      
      if not metadata_directory:
          return None
      metadata_parent = os.path.dirname(metadata_directory)
      if not os.path.isfile(pjoin(metadata_parent, WHEEL_BUILT_MARKER)):
          return None
  
      whl_files = glob(os.path.join(metadata_parent, '*.whl'))
      if not whl_files:
          print('Found wheel built marker, but no .whl files')
          return None
      if len(whl_files) > 1:
          print('Found multiple .whl files; unspecified behaviour. '
                'Will call build_wheel.')
          return None
  
      # Exactly one .whl file
      return whl_files[0]
docs: Check for a wheel already built during the get_wheel_metadata hook.