Move flake-modules to modules2

Move flake-modules to modules2 so they are automatically imported by
import-tree.

modules2/dev-shell.nix

@@ -0,0 +1,14 @@
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      devShells.default = pkgs.mkShell {
+        packages = with pkgs; [
+          just
+          lua-language-server
+          lua54Packages.luacheck
+          nixd
+        ];
+      };
+    };
+}