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.
keybinder-3.0 is for GTK+ 3, whereas keybinder (already in nixpkgs) is
for GTK+ 2.
This adds version 0.3.0 of keybinder-3.0, which we named "keybinder3" in
nixpkgs to (hopefully) lessen the confusion about what is package name
and what is version number.
[Bjørn: extend commit message beyond summary line]
Artturin