Include default package in devShell.inputsFrom

A user whose flake only has a package would expect it to be used when
running `nix develop`. But since theres probably an automatically
created devShell, the package's deps will not be available in the shell.
If a default package exists, it makes sense for its deps to be available
by default in the default devShell.

builtinModules/packages.nix

@@ -98,5 +98,9 @@ in
           packages;
       };
     })
+
+    (mkIf (config.packages ? default) {
+      devShell.inputsFrom = pkgs: [ pkgs.default ];
+    })
   ];
 }