nix-darwin: actually install packages

Also apply assertions when using the nix-darwin module.

Closes #702

(cherry picked from commit 1480a6ca14)

nix-darwin/default.nix

@@ -39,6 +39,22 @@ in
   };
 
   config = mkIf (cfg.users != {}) {
+    assertions =
+      flatten (flip mapAttrsToList cfg.users (user: config:
+        flip map config.assertions (assertion:
+          {
+            inherit (assertion) assertion;
+            message = "${user} profile: ${assertion.message}";
+          }
+        )
+      ));
+
+    users.users = mkIf cfg.useUserPackages (
+      mapAttrs (username: usercfg: {
+        packages = usercfg.home.packages;
+      }) cfg.users
+    );
+
     system.activationScripts.postActivation.text =
       concatStringsSep "\n" (mapAttrsToList (username: usercfg: ''
         echo Activating home-manager configuration for ${username}