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.
freedesktop.org tarballs haven't been published since 0.7.0. I've
sent a message to the mailing list (<87eev3cgcz.fsf@alyssa.is>),
suggesting newer tarballs be published.
Build system has changed from autoconf to meson+cmake/ninja, and the
vtest/util.c patch doesn't seem to be necessary any more.
