PHP 7.3 won't be supported by upstream for the entire life cycle of
the 21.11 release.
Also drop the pcre' alias since it isn't needed anymore since we don't
need different pcre versions anymore.
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.
