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.
Adapted the patches and took a commit from upstream master branch to
allow the migration to Qt5.15.
Had to copy the patch because of the custom sourceRoot.
Jonathan Ringer