opam lint: Warn about unkown variables

[kit-ty-kate]

Some packages use variables such as pkg:var or success or with-test in the build or post-message fields for example, but mistakes might happen and the wrong variable name might be used or there might be some syntax mistake. Would it be possible to check for those variables to see if they might be undefined and raise a warning in that case? They might be custom variables but in most cases they won't be and I think a warning can be useful.

[rjbou]

A warning can be definitely useful, but it would also seems weird as opam permit to check if a variable is defined or not in filters (? operator).

[dra27]

From a review perspective of repository PRs, I imagine it would be handy to have a way for Camelus to list undefined variables so that a person can check that the use looks sound, but I agree that they don't want to be warnings.

[rjbou]

Yes, I see the need from a review perspective. For the moment, all linting check of opam repository are done in opam, but some of them should be separated. Worth adding a way to add lint check via opam-lib. New ones can also be used also in opam-publish.

[rjbou]

digging some issues related to #1801