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/terminal-emulators/rxvt-unicode/default.nix

@@ -1,7 +1,7 @@
 { lib, stdenv, fetchurl, makeDesktopItem
 , libX11, libXt, libXft, libXrender
 , ncurses, fontconfig, freetype
-, pkgconfig, gdk-pixbuf, perl
+, pkg-config, gdk-pixbuf, perl
 , perlSupport      ? true
 , gdkPixbufSupport ? true
 , unicode3Support  ? true
@@ -36,7 +36,7 @@ stdenv.mkDerivation {
 
   buildInputs =
     [ libX11 libXt libXft ncurses  # required to build the terminfo file
-      fontconfig freetype pkgconfig libXrender
+      fontconfig freetype pkg-config libXrender
     ] ++ optional perlSupport perl
       ++ optional gdkPixbufSupport gdk-pixbuf;