Run flake8, cached by stamp_file_name.

def _run_flake8(filename, stamp_file_name, show_lint_files):
    """Run flake8, cached by stamp_file_name."""
    _debug_linter_status("flake8", filename, show_lint_files)
    return _stamped_deps(stamp_file_name,
                         _run_flake8_internal,
                         filename)