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.
These are used to capture the trace without using the GUI tool and
update the saved trace files from older to newer versions. `update` also
allows to re-compress the trace files for whatever reason one might want
to do it.