pandoc-citeproc is an optional dependency of apostrophe's dependency
pypandoc. However apostrophe doesn't use pandoc-citeproc as a filter
(anymore?), so we can safely remove this dependency which invites
breakage as the package has been deprecated by upstream.
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.
