Index the available factories

        Query the plugins for an usable factory and register it

def _register_factory(self):
        """
        Index the available factories

        Query the plugins for an usable factory and register it
        """
        for name, info in self._plugins.items():
            if info['priority']:
                factory = getattr(info['plugin'], 'factory', None)
                if callable(factory):
                    registry[info['factory']] = info['plugin'].factory
        registry.freeze()