Package variables in depexts

[kit-ty-kate]

Currently using package variables such as with-test in the depexts field is authorized but the variable itself seems to be undefined, and the documentation about whether or not package variables are authorized.

I think we should probably:

• make opam lint trigger a warning when an undefined variable is present in the depexts field
• make the documentation clearer that it is not accepted
• think about accepting it in the future in opam 3.0 or something like that if we make opam keep track of the state of package variables (as it currently doesn't)

cc @sim642 who reported about the issue on Discord

[dra27]

I think #5455 is related?