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/os-specific/linux/alsa-tools/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchurl, alsaLib, pkgconfig, gtk2, gtk3, fltk13 }:
+{ lib, stdenv, fetchurl, alsaLib, pkg-config, gtk2, gtk3, fltk13 }:
 # Comes from upstream as as bundle of several tools,
 # some use gtk2, some gtk3 (and some even fltk13).
 
@@ -11,7 +11,7 @@ stdenv.mkDerivation rec {
     sha256 = "0jbkjmq038zapj66a7nkppdf644v2mwj581xbmh6k4i8w6mcglxz";
   };
 
-  nativeBuildInputs = [ pkgconfig ];
+  nativeBuildInputs = [ pkg-config ];
   buildInputs = [ alsaLib gtk2 gtk3 fltk13 ];
 
   patchPhase = ''