Accept any pkg-config --modversion output

[phadej]

This results e.g. in

cabal: The pkg-config package 'blas' version ==0.3 || >0.3 is required but the
version installed on the system is version 0.2.20+ds

output instead of failing to parse 0.2.20+ds as a PkgconfigVersion.
We don't need to parse the pkg-config --modversion output
as strictly as the versions in .cabal file.
I.e. at all, as PkgconfigVersion comparison works somehow
on any possible version strings.

cc @amigalemming if you want to try this out. I'll backport it to 3.2 branch.
You won't be able to write + in .cabal file, but at least Cabal will accept whatever pkg-config --modversion gives it.

[phadej]

Backported in #6542