Change opam packaging to have separate CLI/GUI packages and maybe fsmonitor

[gdt]

Currently, there is only one opam file. I think it requires gtk3 on the host and builds a CLI, but the point is that one should be able to build CLI or GUI as one wishes.

Somewhere between perhaps and probably, have an opam file for fsmonitor (on the set of platforms for which there is an fsmonitor implementation, with sensible behavior on other ones).

While doing this, align with the unison and unison-gui plan in #667.

[gdt]

PR #671 is related.

(My bias is to tread carefully rather than quickly here, but I aim to resolve it all before 2.53.1.)