[General+Flake] convert "default" module directory to subflake

Also move flatpak config from packages to desktop

lib/attrsets.nix

@@ -0,0 +1,17 @@
+{lib, myLib, ...}:
+with lib;
+rec {
+  mergeAttrsRecursive = a: b: foldAttrs (item: acc:
+    if (isNull acc) then
+      item
+    else if (isList item) then
+      if isList acc then
+        acc ++ item
+      else [ acc ] ++ item
+    else if (isString item) then
+      acc + item
+    else if (isAttrs item) then
+      mergeAttrsRecursive acc item
+    else item
+  ) null [ b a ];
+}