Behavior when Remotes are not also listed in other dependencies

[jimhester]

Not entirely sure what the current behavior in pkgdepends is for this, but in devtools if you don't have the Remotes also listed in one of the other fields it is silently skipped.

I now think we may want to either install them regardless or throw an error in this case, but silently ignoring them just leads to confusion.

[gaborcsardi]

I don't think pkgdepends should fail, because this might happen downstream, and then it will be very annoying if you need a package that you don't maintain, and thus cannot fix. pkgdepends should install them, anyway, I think.

I think devtools::load_all() or desc should fail/warn about this, then the package maintainer is targeted.

[jimhester]

remotes now installs these packages anyway, which I think we should probably do in pak / pkgdepends as well