### Godot version

4.0.3.stable

### System information

Linux/X11

### Issue description

when creating addons, i want to use relative paths so that things don't break if i (or another user) decides to rename and/or move the add-on's folder.

using relative paths with ``add_autoload_singleton()`` doesn't work because of an error when enabling the add-on.

## ``res://addons/my_plugin/plugin.gd``

```GDScript
@tool
extends EditorPlugin


func _enter_tree():
	# this line doesn't work:
	add_autoload_singleton('MyAutoload', 'my_autoload.gd')
	# these lines work:
	#add_autoload_singleton( 'MyAutoload',
	#	'res://addons/my_plugin/my_autoload.gd'
	#)


func _exit_tree():
	remove_autoload_singleton('MyAutoload')
```

## error message window when enabling the add-on

```
┌─────────────────────────────────────────────────┐
│ Warning!                                  _ □ X │
├─────────────────────────────────────────────────┤
│ Can't add Autoload:                             │
│ res:// is an invalid path. File does not exist. │
│                                                 │
│              [OK]      [Copy Text]              │
└─────────────────────────────────────────────────┘
```

the add-on does not get disabled because of the error.
the error-free parts of the add-on still get activated, just the autoload doesn't get created. 

### Steps to reproduce

1. create a plugin with the subfolder "my_plugin" and script name "plugin.gd"
2. create an empty script in ``res://addons/my_plugin/my_autoload.gd``
3. edit ``res://addons/my_plugin/plugin.gd``:
```GDScript
@tool
extends EditorPlugin


func _enter_tree():
	add_autoload_singleton('MyAutoload', 'my_autoload.gd')


func _exit_tree():
	remove_autoload_singleton('MyAutoload')

```
4. enable the plugin
5. see the error message window

### Minimal reproduction project

[add_autoload_singleton.zip](https://github.com/godotengine/godot/files/11706746/add_autoload_singleton.zip)
(13.9 KiByte download, 11.4 KiByte unzipped)
return EditorNode::get_singleton()->get_export_as_menu();
}
void EditorPlugin::set_input_event_forwarding_always_enabled() {
	input_event_forwarding_always_enabled = true;
	EditorPluginList *always_input_forwarding_list = EditorNode::get_singleton()->get_editor_plugins_force_input_forwarding();
	always_input_forwarding_list->add_plugin(this);
}
void EditorPlugin::set_force_draw_over_forwarding_enabled() {
	force_draw_over_forwarding_enabled = true;
	EditorPluginList *always_draw_over_forwarding_list = EditorNode::get_singleton()->get_editor_plugins_force_over();
	always_draw_over_forwarding_list->add_plugin(this);
}
void EditorPlugin::notify_scene_changed(const Node *scn_root) {
	emit_signal(SNAME("scene_changed"), scn_root);
}
void EditorPlugin::notify_main_screen_changed(const String &screen_name) {
	if (screen_name == last_main_screen_name) {
		return;
	}
	emit_signal(SNAME("main_screen_changed"), screen_name);
	last_main_screen_name = screen_name;
}
void EditorPlugin::notify_scene_closed(const String &scene_filepath) {
	emit_signal(SNAME("scene_closed"), scene_filepath);
}
void EditorPlugin::notify_resource_saved(const Ref<Resource> &p_resource) {
	emit_signal(SNAME("resource_saved"), p_resource);
}
bool EditorPlugin::forward_canvas_gui_input(const Ref<InputEvent> &p_event) {