Add opam-switch-mode to lang: coq and fix configuration of opam-switch-mode in lang: ocaml

[jeremyThibault]

This PR adds configuration to install and use opam-switch-mode to manage opam switches to the coq module. The configuration is taken almost straight from PR #7806 which added the same capability to the ocaml module. The only difference is that, unlike in the ocaml module, we do not initialize the environment right away.
Incidentally, we need to wrap this initialization inside the ocaml module, or it will try to execute even if tuareg isn't loaded and produce an error.

Note that this is my first PR, and that I am not very familiar with Emacs and use-package, so I'm not very confident that commit bdb4d60 is doing the right thing; at least, it seems to work on my machine.
I'm also not sure if it's fine to use the same package! in two different, non-mutually-exclusive modules.

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