libungif was merged into giflib in 2006, and hasn't been updated
since. All non-broken packages still using it build fine with giflib.
See <http://giflib.sourceforge.net/history.html>.
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.
pslib is a C-libreary to create PostScript files.
If applied, other nix package (e.g. the in-progress GNU Data Language)
with such dependency can be packaged easier.
