monorepo: support pinning ocaml-variants

[emillon]

Hi,

We're using opam-monorepo to test multicore branches against JS releases. The idea is that it's quicker to iterate when no opam pinning is involved. The workflow is described here.

This is working well but iterating locally on a fix relies on creating compiler from branches. I'm using opam-compiler to do this, but it would help to do that directly in CI.

The workflow would be like the following:

• add a pin-depends entry for ocaml-variants in the source opam file (possible improvement: support the same syntax as opam-compiler create so one can select ocaml-multicore/ocaml-multicore#123 as source)
• opam monorepo lock propagates that info to the lock file
• when ocaml-ci sees such a lock file, it will create a switch based on that source specification and proceed to pull the packages as usual