qt5: factor out common definitions and remove symlink farm

The module definitions are factored out and shared between qt56 and qt59. The
symlink farm which was created during builds is no longer needed.

pkgs/development/libraries/qt-5/modules/qtmacextras.nix

@@ -0,0 +1,10 @@
+{ qtModule, qtbase, lib }:
+
+qtModule {
+  name = "qtmacextras";
+  qtInputs = [ qtbase ];
+  meta = with lib; {
+    maintainers = with maintainers; [ periklis ];
+    platforms = platforms.darwin;
+  };
+}