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/conky/default.nix

@@ -1,4 +1,4 @@
-{ config, lib, stdenv, fetchFromGitHub, pkgconfig, cmake
+{ config, lib, stdenv, fetchFromGitHub, pkg-config, cmake
 
 # dependencies
 , glib, libXinerama
@@ -89,7 +89,7 @@ stdenv.mkDerivation rec {
 
   NIX_LDFLAGS = "-lgcc_s";
 
-  nativeBuildInputs = [ cmake pkgconfig ];
+  nativeBuildInputs = [ cmake pkg-config ];
   buildInputs = [ glib libXinerama ]
     ++ optionals docsSupport        [ docbook2x docbook_xsl docbook_xml_dtd_44 libxslt man less ]
     ++ optional  ncursesSupport     ncurses