### 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)
Ref<Texture2D> icon;
	GDVIRTUAL_CALL(_get_plugin_icon, icon);
	return icon;
}
bool EditorPlugin::has_main_screen() const {
	bool success = false;
	GDVIRTUAL_CALL(_has_main_screen, success);
	return success;
}
void EditorPlugin::make_visible(bool p_visible) {
	GDVIRTUAL_CALL(_make_visible, p_visible);
}
void EditorPlugin::edit(Object *p_object) {
	GDVIRTUAL_CALL(_edit, p_object);
}
bool EditorPlugin::handles(Object *p_object) const {
	bool success = false;
	GDVIRTUAL_CALL(_handles, p_object, success);
	return success;
}
Dictionary EditorPlugin::get_state() const {
	Dictionary state;
	GDVIRTUAL_CALL(_get_state, state);
	return state;
}
void EditorPlugin::set_state(const Dictionary &p_state) {