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.
For Linux, the phases was run in the wrong order. I've
fixed that, but the package is still a complete mess that
needs to be cleaned up. The linux and darwin builds should
probably be separated into two different Nix expressions
to avoid the current conditional mess.
