add initial modules lib

lib/modules.nix

@@ -0,0 +1,19 @@
+{ inputs, pkgs, lib, ... }:
+
+let
+
+in {
+  mkBoolOpt = { def, desc ? "" }:
+  {
+    type = lib.types.bool;
+    default = def;
+    description = desc;
+  };
+
+  linkMutable = { src, dest, after }:
+  lib.hm.dag.entryAfter [ "writeBoundary" ] ++ after ''
+    if [ ! -L "${dest}" ]; then
+      ln -sf ${src} ${dest}
+    fi
+  '';
+}