opam lint should raise a warning when the license field is missing

[kit-ty-kate]

Mentioned in ocaml/opam-repository#18343

It would be also nice to have (if there is not already one) an escape hatch for more complex licenses that don't fit in the SPDX standard. I would suggest something like license: "custom:<custom license name>"

[dra27]

Cross-referencing #4686 (comment)

[dra27]

Do we definitely want an escape hatch - having gone to the trouble of getting things like the OCaml linking exception text ratified and added to SPDX (see #2224), is it too bad for us to make it a lint warning with no escape?

[kit-ty-kate]

I'm currently making an SPDX parser to add to a separate PR to #4766. It turns out the SPDX format already has a way to add custom licenses, e.g. LicenseRef-my-special-license (https://spdx.github.io/spdx-spec/appendix-V-using-SPDX-short-identifiers-in-source-files/).

However I prefer to split the two features, as currently it's not such a big deal to not have support for custom licenses. We already ignore the warnings in opam-repo-ci when needed and we can have both at different times and even backport one if it's really necessary. The SPDX parser feature is larger chunk of work so I also prefer to not slow down the addition of feature actively asked by users.

[dra27]

All good - I definitely agree with getting the missing check on license straight in, too