code: def _prepare_vts_results_dir(self, vts):
      
      vt_set_results_dir = os.path.join(self.versioned_workdir, 'results', vts.cache_key.hash)
      safe_mkdir(vt_set_results_dir)
      return vt_set_results_dir
docs: Given a `VergetTargetSet`, prepare its results dir.