Currently depexts availability is calculated on demand on the platforms where the command to do it, do it in a reasonable amount of time (e.g. see #6426 #4548 and #5372).

Proposal: to allow most (all?) supported platforms to finally get on the same level in terms of depexts feature, we could instead move the availability calculation during opam update, during repository cache reload to be more precise. Doing it during cache reload instead of unconditionally during opam update would allow users to actually experience #6283 whenever we merge this.

The reasoning behind this is that the set of available system packages don't really change all that often (new packages are slowly added and removed) and such change would be tied to a change in the repository if used anyway. Also use of depexts in vcs/local pins is extremely rare.

The installed depexts calculation is usually much faster and easier to do so this can stay in the same place.

Doing so should allow all platforms to finally get on par with each other, in particular regarding the virtual packages support (see #4759)