def _add_default_source(self):
        """Add the package's default configuration settings. This looks
        for a YAML file located inside the package for the module
        `modname` if it was given.
        """
        if self.modname:
            if self._package_path:
                filename = os.path.join(self._package_path, DEFAULT_FILENAME)
                if os.path.isfile(filename):
                    self.add(ConfigSource(load_yaml(filename), filename, True))
Add the package's default configuration settings. This looks
        for a YAML file located inside the package for the module
        `modname` if it was given.