Lint check use of package variables in depends/depopts/conflicts/depexts

[kit-ty-kate]

Currently package variables are not allowed in the depends, depopts, conflicts or depexts field (see #5455 for the latter). However opam doesn't warn of their uses in these fields which confuses users (see ocaml/opam-repository#25863)

It would be nice to have them trigger a warning during linting.
It would be nice to also mention this in the documentation in the section on package variables. As far as i know, the only mention of it in the documentation is in the description of the depends field

[kit-ty-kate]

As noted by @rjbou, W29 does that already but only misses depexts

[lukstafi]

Note that it is not unreasonable to think that depexts might have more lenient restrictions than depends, since they cannot depend on opam packages or have exclusion conflicts.