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.
Built this locally and noticed that the hash changed
wanted: sha256:0x5vj5zfk95avyg6g3nf6gar250cfrgla2ixj2ifn8pcick2d9vq
got: sha256:0m7rxv2nnycvwqpc8ak396g8cbb9g6dw290cjbfwhrrhdhbii5qc
