Add warnings for unsupported distributions when failing to resolve a dependency

[mosteo]

Discussed in #1997

^{Originally posted by carrexxii August 10, 2025}
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]

In essence:

• If dep is external and distro is unknown, inform that that will always fail, instruct on using a "compatible" distro.
• If dep is external without definition for the distro, inform that the dependency should be installed manually and the error is expected. (Something similar may be already happening, double check).