Offer uninstalled compiler as fallback with warning (#1508)

Offer uninstalled compiler as fallback with warning (#1508)

* Allow solving with unistalled tools as fallback

* Fix precedence of installed compiler over remote ones

* Add warning in solution diff about toolchain download

* Fallback on installed compiler over uninstalled one

This is after checking that the preferred compiler cannot fulfill the compiler
dependency.

* Tweaks to restore former behaviors

* Fix test that inadvertently used virtual crate

It should have been real all along without alternatives

* Rebase tweaks

* Reword information message