Add warnings for unsupported distributions when failing to resolve a dependency

[carrexxii]

I think a warning should be added when alr install fails to find all dependencies and the distribution is unknown. When trying to install libadalang, I had it reporting that it couldn't find libgmp without it providing any other information. I didn't realise that Alire uses the package manager to resolve dependencies (or failed to in this case) and only stumbled across it in the settings for distribution.override.

I'm using Artix and the solution was a trivial as --set distribution.override arch to get it to use pacman and it would be good if there were a warning indicating an unknown distribution so that in the future others in my situation can find the issue more easily.

[mosteo]

Hints were supposed to help with that, but they're on the way out due to internal complexity and confusion reported by some users.

Do you remember how libgmp was reported specifically in the solution?

[carrexxii]

The error was Dependencies (missing): error: libgmp* (indirect,hinted).

The meaning of hinted is honestly more confusing to me now having read the comment "Undetected external" because that seems to say the same thing as missing.