treewide: pkgs.pkgconfig -> pkgs.pkg-config, move pkgconfig to alias.nix

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.

pkgs/applications/window-managers/afterstep/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, pkgconfig
+{ lib, stdenv, fetchurl, pkg-config
 , libjpeg, libtiff, libpng, freetype
 , fltk, gtk
 , libX11, libXext, libICE
@@ -24,10 +24,10 @@ stdenv.mkDerivation rec {
     })
   ];
 
-  nativeBuildInputs = [ pkgconfig ];
+  nativeBuildInputs = [ pkg-config ];
   buildInputs = [ libjpeg libtiff libpng freetype fltk gtk libX11 libXext libICE dbus dbus ];
 
-  # A strange type of bug: dbus is not immediately found by pkgconfig
+  # A strange type of bug: dbus is not immediately found by pkg-config
   preConfigure = ''
      export NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE $(pkg-config dbus-1 --cflags)"
   '';