docs: def validate_args(self):
          """Input validators for this rule type."""
          base.BaseTarget.validate_args(self)
          params = self.params
          if params['extra_control_fields'] is not None:
              assert isinstance(params['extra_control_fields'], list), (
                  'extra_control_fields must be a list of tuples, not %s' % type(
                      params['extra_control_fields']))
              for elem in params['extra_control_fields']:
                  assert (isinstance(elem, tuple) and len(elem) == 1), (
                      'extra_control_fields must be a list of 2-element tuples. '
                      'Invalid contents: %s' % elem)
          pkgname_re = '^[a-z][a-z0-9+-.]+'
          assert re.match(pkgname_re, params['package_name']), (
              'Invalid package name: %s. Must match %s' % (
                  params['package_name'], pkgname_re))