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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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))