docs: function urlFromIndex(index) {
      assert.bool(index.official, 'index.official');
      assert.optionalString(index.scheme, 'index.scheme');
      assert.string(index.name, 'index.name');
  
      if (index.official) {  // v1
          return DEFAULT_INDEX_URL;
      } else {
          return fmt('%s://%s', index.scheme || 'https', index.name);
      }
  }