'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.'

def _add_default_source(self):

    if self.modname:
        pkg_path = _package_path(self.modname)
        if pkg_path:
            filename = os.path.join(pkg_path, DEFAULT_FILENAME)
            if os.path.isfile(filename):
                self.add(ConfigSource(load_yaml(filename), filename, True))
