`cabal-install` could print every missing library that is missing from `pkg-config` in one go

I recently provisioned a big app on a new machine and it took several iterations of `cabal build` before it complained about every missing library in `pkg-config`. I am not familiar with how the solver knows things, but could it not print one message with *all* the missing libraries?

cc @grayjay (you are my solver expertjl)

Start by reproducing the repeated `cabal build` behavior with a project that has multiple missing libraries in `pkg-config`, then trace how the solver reports missing libraries. Done means `cabal-install` reports all missing libraries in one run, with regression coverage for the diagnostic.

Written by the indexing model from the issue text.