Create nixfiles lib and define flake type

lib/nixfiles/default.nix

@@ -0,0 +1,7 @@
+pkgs:
+let
+  inherit (pkgs) lib;
+in
+{
+  types = (import ./types.nix) pkgs;
+}

lib/nixfiles/types.nix

@@ -0,0 +1,13 @@
+pkgs:
+let
+  inherit (pkgs) lib;
+  inherit (lib.types) mkOptionType;
+  inherit (lib.options) mergeEqualOption;
+in
+{
+  flake = mkOptionType {
+    name="flake";
+    description="Nix flake";
+    descriptionClass = "noun";
+    merge = mergeEqualOption; check = (value: value._type or "" == "flake"); };
+}