Get packages involved in a failure.

        Args:
            failure_index: See `failure_reason`.

        Returns:
            A list of Requirement objects.

def failure_packages(self, failure_index=None):
        """Get packages involved in a failure.

        Args:
            failure_index: See `failure_reason`.

        Returns:
            A list of Requirement objects.
        """
        phase, _ = self._get_failed_phase(failure_index)
        fr = phase.failure_reason
        return fr.involved_requirements() if fr else None