`--deps-only` should install the depexts of the package too

[AltGr]

In the expected workflow, one opam switch create . --deps-only command should be enough to setup a dev environment. However, that skips the depexts of the local packages at the moment, which defeats some expectations.

I guess this could stay unnoticed until now since in general the depexts should be done through conf- packages; but sometimes you need a more custom setup.

I don't think enabling this by default should cause issues; but if that was to be the case, at least we should have an flag to get this behaviour (maybe a way of combining --deps-only and --depext-only ?)