Callback for _check_repos.

def _repos_checked(self, worker, output, error):
        """Callback for _check_repos."""
        if worker.repo in self._checking_repos:
            self._checking_repos.remove(worker.repo)

        if output:
            self._valid_repos.append(worker.repo)

        if len(self._checking_repos) == 0:
            self._download_repodata(self._valid_repos)