Check the compiler version when pulling

[Leonidas-from-XIV]

When opam-monorepo pull is called it just pulls the tarballs from the lock file. What it does not do is to check the OCaml version, which can lead to issues where the locked packages require a newer compiler than is available in the switch, thus failing to compile with a syntax error of whatever code failed to compile on the current switch.

Since the ocaml-base-compiler version is noted in the lockfile, we could show an error/warning if the version of the compiler in the current switch does not match what is in the lockfile to improve errors.