`plots`: don't work unless the `stages` key is also in `dvc.yaml`.

[dberenbaum]

    We discussed this already @pared, but for the record here, `plots` don't work unless the `stages` key is also in `dvc.yaml`.

Originally posted by @dberenbaum in #7477 (comment)

[dberenbaum]

We are also discussing introducing top-level params and metrics so all of these can be configured without stages. It will be important that they can work even if the users have no pipeline stages. cc @dtrifiro @daavoo

[dberenbaum]

Fixed by #8529