<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _find_already_built_wheel(metadata_directory):
 """Check for a wheel already built during the get_wheel_metadata hook. """

    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]