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.
Because nobody told me how to add the missing cargo-metadata command to
the test environment and the PR for updating this package is almost
half a year old, disable tests now.
Hopefully someone will step up and fix this, but I doubt it.
Signed-off-by: Matthias Beyer <mail@beyermatthias.de>
