When solving, we now discard plans that would involve packages with a
pkgconfig-depends constraint which is not satisfiable with the current
set of installed packages (as listed by pkg-config --list-all).

This fixes https://github.com/haskell/cabal/issues/3016.

It is possible (in principle, although it should be basically impossible
in practice) that "pkg-config --modversion pkg1 pkg2... pkgN" fails to
execute for various reasons, in particular because N is too large, so
the command line becomes too long for the operating system limits.

If this happens, revert to the previous behavior of accepting any
install plan, regardless of any pkgconfig-depends constraints.