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.
Fixes#98769.
Important changes from https://gstreamer.freedesktop.org/releases/1.18/:
* `gst-validate` was renamed to `gst-devtools` upstream:
> * the `gst-validate` tarball has been superseded by
> the `gst-devtools` tarball for consistency with the git module name.
* `gst-python` is now Python 3 only:
> * Python 2.x is no longer supported
