continuation of #109595
pkgconfig was aliased in 2018, however, it remained in
all-packages.nix due to its wide usage. This cleans
up the remaining references to pkgs.pkgsconfig and
moves the entry to aliases.nix.
python3Packages.pkgconfig remained unchanged because
it's the canonical name of the upstream package
on pypi.
This is used in https://github.com/tweag/gomod2nix to reconstruct a
vendor metadata file.
With the vendor checks we need a lot more metadata which isn't
relevant for building packages, especially since we've already locked
the dependency graph ahead of time
This has been ported from FreeBSD: https://reviews.freebsd.org/D24122
