docs: def _bind_service(package_name, cls_name, binder=Injected):
          """Bind service to application injector.
  
          :param package_name: service package
          :type package_name: str
          :param cls_name: service class
          :type cls_name: str
          :param binder: current application binder, injected
          :type binder: Binder
          """
          module = importlib.import_module(package_name)
          cls = getattr(module, cls_name)
  
          binder.bind(
              cls,
              to=binder.injector.create_object(cls),
              scope=singleton
          )
          logging.debug("Created {0} binding.".format(cls))