Add lint for x-maintenance-intent

[shonfeder]

It should validate that the format is correct when present. We'd also like a warning (not a failure) when it is absent. See https://github.com/ocaml/opam-repository/blob/master/governance/policies/archiving.md#specification-of-the-x--fields-used-in-the-archiving-process for the format.

See also @raphael-proust's https://github.com/raphael-proust/maintenance-intent-filter . We should share that code if possible.

[hannesm]

I would appreciate this becoming reality :)

[shonfeder]

@hannesm where is currently the canonical source of truth for the valid format? Is there already re-usable code to validate it we can plug in here?

[hannesm]

@shonfeder sorry, I don't have a "canonical source of truth for the valid format". https://github.com/hannesm/maintenance-intent-filter/blob/develop/mintent.ml is what I use, based on @raphael-proust code https://github.com/raphael-proust/maintenance-intent-filter , the extension is to support "(none)" as well.

[shonfeder]

We discussed in the opam maintainers meeting today, and the aim is to publish https://github.com/raphael-proust/maintenance-intent-filter, with a check function, which we can then plug in to the linter.