Add lint check for archive repo

[shonfeder]

As per the archiving plan we need a lint check that will

• checks that every dependency constraint is of the form "dep" {... & <= ""} except for "ocaml" (needed for phase 2)
• checks if the x-reason-for-archival is set (needed for phase 3)
• checks if the x-opam-repository-commit-hash-at-time-of-archival is set (needed for phase 3)

in accordance with the archiving policy

We could do this in a standalone tool, but since the purpose of opam-ci-check is to support the linting requirements for the opam repo, and this is an extension of that, I think it is the right place.

Once #412 is merged, I'll just need to update the Git workflow at https://github.com/ocaml/opam-repository-archive/blob/04de824d1d8e75e729440d214181556fc613e7b6/.github/workflows/main.yml#L34 to use the live branch of ocurrent.