Unify explicit_repos_solver with local_opam_config_solver

[Leonidas-from-XIV]

The idea would be to read all the relevant configuration from the local switch state and pass it to the explicit_repos_solver and stop using the local_opam_config_solver eventually.

The advantage is that changes to OPAM variables, repos etc. can be made explicitly by opam-monorepo at runtime, instead of having to modify the switch or global state (thus avoiding the "please add the opam-overlays repo to your switch" message, since opam-monorepo can do it by itself without clobbering the users switch).

Depends on #284, since it requires the explicit_repos_solver to work with external repos like opam-repository.

[tmattio]

Related to #333.