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

Also move flatpak config from packages to desktop

lib/types.nix

@@ -0,0 +1,11 @@
+{ lib, myLib }:
+with lib;
+{
+  package' = types.package // {
+    merge = loc: defs:
+      let res = mergeDefaultOption loc defs;
+      in if builtins.isPath res || (builtins.isString res && ! builtins.hasContext res)
+        then toDerivation res
+        else res;
+  };
+}