### 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)
bool success = false;
	GDVIRTUAL_CALL(_forward_canvas_gui_input, p_event, success);
	return success;
}
void EditorPlugin::forward_canvas_draw_over_viewport(Control *p_overlay) {
	GDVIRTUAL_CALL(_forward_canvas_draw_over_viewport, p_overlay);
}
void EditorPlugin::forward_canvas_force_draw_over_viewport(Control *p_overlay) {
	GDVIRTUAL_CALL(_forward_canvas_force_draw_over_viewport, p_overlay);
}
int EditorPlugin::update_overlays() const {
	if (Node3DEditor::get_singleton()->is_visible()) {
		int count = 0;
		for (uint32_t i = 0; i < Node3DEditor::VIEWPORTS_COUNT; i++) {
			Node3DEditorViewport *vp = Node3DEditor::get_singleton()->get_editor_viewport(i);
			if (vp->is_visible()) {
				vp->update_surface();
				count++;
			}
		}
		return count;
	} else {
		
		CanvasItemEditor::get_singleton()->get_viewport_control()->queue_redraw();
		return 1;
	}
}
EditorPlugin::AfterGUIInput EditorPlugin::forward_3d_gui_input(Camera3D *p_camera, const Ref<InputEvent> &p_event) {