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.
`updateScript` lets us update packages automatically.
For packages that are hosted on the gnome mirror it's very easy to add
that. Inspired by https://github.com/NixOS/nixpkgs/issues/36150
Libunique is a library to write single instance applications.
I am creating a new package instead of upgrading the old one because
Mate uses libunique 3.x and other projects not (as far as I know).
The one in gnome2 was failing to build,
but all there is likely in a desolate state anyway.
In gmpc it also seemed without any reason to have a duplicate.
