Use pkg_resources to find the path to the required resource.

def Filter(self, filename_spec):
    """Use pkg_resources to find the path to the required resource."""
    if "@" in filename_spec:
      file_path, package_name = filename_spec.split("@")
    else:
      file_path, package_name = filename_spec, Resource.default_package

    resource_path = package.ResourcePath(package_name, file_path)
    if resource_path is not None:
      return resource_path


    # pylint: disable=unreachable
    raise FilterError(
        "Unable to find resource %s while interpolating: " % filename_spec)