fix opam-monorepo issue with Ocaml5+

[palainp]

Dear developers,

This PR should solve tarides/opam-monorepo#340 and permits to compile mirage unikernels with Ocaml5+.

The underlying issue was the ocaml package marked as opam-provided and thus removed from dependencies at https://github.com/tarides/opam-monorepo/blob/9cc963ce7bc91592a301feca1e2238bc17448bd8/lib/opam_solve.ml#L114 (This can be seen inside the solver: ocaml-opam/opam-0install-solver#47). Therefore, as https://github.com/ocaml/opam-repository/blob/master/packages/base-domains/base-domains.base/opam have a dependency list which contains ocaml OR ocaml-variants, leave the solver only with ocaml-variants if we remove ocaml.

For the time being I moved ocaml to the Monorepo scope, this is why it now appears as monorepo in the test files, but unsure if that the best move, comments are more than welcome!

[hannesm]

Thanks for the investigations. What about removing ocaml dependency from the generated opam file completely (since e.g. mirage-runtime depends on it, we will have an implicit dependency on it anyways) -- since we use ocaml-solo5 for cross-compilation (when choosing a solo5 target), there's not really a need for the ocaml dependency anyways (and we don't quite need it in the monorepo).

What do others think? I'm not too knowledgeable about the opem-monorepo and 0install-solver stuff.

[palainp]

Thanks for your reply @hannesm!
To me your last argument with the cross-compilation wins all (in the future cross-compilation may also be used for other targets)!

I'll add a commit to remove ocaml from the dependecies, so far I managed to compile q-m-f with success.

[hannesm]

great, thanks