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

Also move flatpak config from packages to desktop

lib/config.nix

@@ -0,0 +1,27 @@
+{lib, myLib}:
+with lib;
+let
+  mkListToString = { sep ? " " }: list: concatStringsSep sep (
+    forEach list (v: toString v)
+  );
+in rec {
+  mkValueString =
+    let
+      gen = generators.mkValueStringDefault {};
+      listToString = mkListToString {};
+    in v: if isList v then listToString v
+          else gen v;
+  
+  mkKeyValue = { sep }: with generators; toKeyValue {
+    mkKeyValue = mkKeyValueDefault {
+      mkValueString = mkValueString;
+    } sep;
+  };
+
+  toSystemd = mkKeyValue {
+    sep = "=";
+  };
+  toNix = mkKeyValue {
+    sep = " = ";
+  };
+}