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.
Since we now use qt5.15 in all python modules (defaulting to the same
qt5 used in all-packages.nix), That patch is not required, but it will
be needed if qt5.14 is used if pythonInterpreters is overridden, which
is likely to happen like it happend to qutebrowser for instance. See:
2667af4062
