Move flake-modules to modules2

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

modules2/packages.nix

@@ -0,0 +1,17 @@
+{ self, ... }:
+
+{
+  perSystem =
+    { inputs', pkgs, ... }:
+    {
+      packages = {
+        default = pkgs.mkShell { buildInputs = with pkgs; [ just ]; };
+
+        neovim = inputs'.nixvim.legacyPackages.makeNixvimWithModule {
+          inherit pkgs;
+
+          module = import "${self}/modules/home-manager/neovim/config";
+        };
+      };
+    };
+}