Make package manager configurable (particularly on archlinux)

[Gbury]

(This is a feature request)

Would it be possible to make it so that one can configure the system package manager that opam uses for depexts ?

Context: On Archlinux, it happens that a few packages are not in the official repositories, but on a user-managed database of packages called the AUR. Although pacman -- the official package manager of archlinux -- cannot install packages from the AUR, there are a lot of AUR helpers/pacman wrappers which can replace pacman and allow users to directly install packages from the AUR. Using an AUR helper/pacman wrapper is extremely common among archlinux users, at least to my knowledge, especially since most of them actually expand on pacman's CLI. Therefore, it might be useful to allow users to configure the binary/package manager used by opam (at their depends if the new binary does not have the same CLI as the original package manager) so that for instance, depexts can be installed from the AUR without manual intervention.