def fixup_namespace_packages(path_item, parent=None):
    """Ensure that previously-declared namespace packages include path_item"""
    _imp.acquire_lock()
    try:
        for package in _namespace_packages.get(parent, ()):
            subpath = _handle_ns(package, path_item)
            if subpath:
                fixup_namespace_packages(subpath, package)
    finally:
        _imp.release_lock()
Ensure that previously-declared namespace packages include path_item
src/pkg_resources/__init__.py