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/top-level/emscripten-packages.nix

@@ -10,7 +10,7 @@ rec {
     stdenv = pkgs.emscriptenStdenv;
   }).overrideDerivation
     (old: {
-      nativeBuildInputs = [ autoreconfHook pkgconfig ];
+      nativeBuildInputs = [ autoreconfHook pkg-config ];
       propagatedBuildInputs = [ zlib ];
       buildInputs = old.buildInputs ++ [ automake autoconf ];
       configurePhase = ''
@@ -49,7 +49,7 @@ rec {
   }).overrideDerivation
     (old: { 
       propagatedBuildInputs = [ zlib ];
-      buildInputs = old.buildInputs ++ [ pkgconfig ];
+      buildInputs = old.buildInputs ++ [ pkg-config ];
 
       # just override it with nothing so it does not fail
       autoreconfPhase = "echo autoreconfPhase not used..."; 
@@ -84,8 +84,8 @@ rec {
     pname = "xmlmirror";
     version = "unstable-2016-06-05";
 
-    buildInputs = [ pkgconfig autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
-    nativeBuildInputs = [ pkgconfig zlib ];
+    buildInputs = [ pkg-config autoconf automake libtool gnumake libxml2 nodejs openjdk json_c ];
+    nativeBuildInputs = [ pkg-config zlib ];
 
     src = pkgs.fetchgit {
       url = "https://gitlab.com/odfplugfest/xmlmirror.git";
@@ -136,7 +136,7 @@ rec {
     stdenv = pkgs.emscriptenStdenv;
   }).overrideDerivation
     (old: { 
-      buildInputs = old.buildInputs ++ [ pkgconfig ];
+      buildInputs = old.buildInputs ++ [ pkg-config ];
       # we need to reset this setting!
       NIX_CFLAGS_COMPILE="";
       configurePhase = ''