code: def failure_packages(self, failure_index=None):
          
          phase, _ = self._get_failed_phase(failure_index)
          fr = phase.failure_reason
          return fr.involved_requirements() if fr else None
docs: Get packages involved in a failure.
  
          Args:
              failure_index: See `failure_reason`.
  
          Returns:
              A list of Requirement objects.