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.
Presently this means static, instead of the shared we started
building after a recent change (my blame).
Darwin build breaks with this,
and it was introduced on a whim so instead of making this
platform-specific, go back to how things were previously.
