code: def _checkout(self, project):
          
          if len(self.version) != 1:
              dbt.exceptions.raise_dependency_error(
                  'Cannot checkout repository until the version is pinned.')
          try:
              dir_ = dbt.clients.git.clone_and_checkout(
                  self.git, DOWNLOADS_PATH, branch=self.version[0],
                  dirname=self._checkout_name)
          except dbt.exceptions.ExecutableError as exc:
              if exc.cmd and exc.cmd[0] == 'git':
                  logger.error(
                      'Make sure git is installed on your machine. More '
                      'information: '
                      'https://docs.getdbt.com/docs/package-management'
                  )
              raise
          return os.path.join(DOWNLOADS_PATH, dir_)
docs: Performs a shallow clone of the repository into the downloads
          directory. This function can be called repeatedly. If the project has
          already been checked out at this version, it will be a no-op. Returns
          the path to the checked out directory.