check opam switch - compatible

[ylecornec]

See #11570 for the develop version of this PR.

This PR enables the check_opam_switch tool, that provides two binaries to check that the current opam switch contains the packages from the opam.export file (at the same version):

• check_opam_switch simply does the check,
• dune_wrapper does the check before invoking dune (so it is possible to keep using dune as usual (with an alias) but notice early when the switch needs to be updated).

Other changes:

• The opam.export file was moved to the root of the dune project, as expected by dune_wrapper by default (this way it is simpler to use dune_wrapper it in multiple projects).

• The check was added to the ocaml_checks rule of the makefile (to notice early that the switch needs to be updated when compiling the project from the makefile).

See issue #10982 and PR #11450 for context.