templates: Handle package.json conversion in direnv

By splitting out the check for package.yaml into a separate check, we
can avoid the constant massive shell reloads for the nix shell.

flake.nix

@@ -34,17 +34,6 @@
             yj
           ]
           ++ old.buildInputs;
-
-        shellHook =
-          ''
-            # Update package.json
-            if [ -e ./package.json ]; then
-                unlink ./package.json
-            fi
-
-            cat ./package.yaml | yj > ./package.json
-          ''
-          + old.shellHook;
       });
 
       server = nixpkgs.legacyPackages.${system}.mkShell {

templates/.envrc

@@ -0,0 +1,10 @@
+nix_direnv_watch_file package-lock.json
+use flake ..#templates
+
+# Update package.json
+if ! cmp --silent .checksum-package.yaml <(sha256sum package.yaml); then
+    echo "Updating package.json"
+    unlink ./package.json
+    yj < ./package.yaml > ./package.json
+    sha256sum package.yaml > .checksum-package.yaml
+fi

templates/.gitignore

@@ -3,3 +3,4 @@ dist/
 node_modules/
 package.json
 result
+.checksum-package.yaml