Check the package universe remains closed.

[dbuenzli]

Dear @camelus !

We need your help to make sure that these kind of issue do not pop up in the OCaml opam repository.

In other words {test} and {dev} (and others ?) dependency names should be checked to exist in the repo on opam submissions.

[AltGr]

Heard. I was planning at some point to have another level of linting, that is aware of the package universe: it could be used to check package variables too, for example. This would be a good opportunity for such a check.