### 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)
ERR_FAIL_COND(!p_gizmo_plugin.is_valid());
	Node3DEditor::get_singleton()->add_gizmo_plugin(p_gizmo_plugin);
}
void EditorPlugin::remove_node_3d_gizmo_plugin(const Ref<EditorNode3DGizmoPlugin> &p_gizmo_plugin) {
	ERR_FAIL_COND(!p_gizmo_plugin.is_valid());
	Node3DEditor::get_singleton()->remove_gizmo_plugin(p_gizmo_plugin);
}
void EditorPlugin::add_inspector_plugin(const Ref<EditorInspectorPlugin> &p_plugin) {
	ERR_FAIL_COND(!p_plugin.is_valid());
	EditorInspector::add_inspector_plugin(p_plugin);
}
void EditorPlugin::remove_inspector_plugin(const Ref<EditorInspectorPlugin> &p_plugin) {
	ERR_FAIL_COND(!p_plugin.is_valid());
	EditorInspector::remove_inspector_plugin(p_plugin);
}
void EditorPlugin::add_scene_format_importer_plugin(const Ref<EditorSceneFormatImporter> &p_importer, bool p_first_priority) {
	ERR_FAIL_COND(!p_importer.is_valid());
	ResourceImporterScene::add_importer(p_importer, p_first_priority);
}
void EditorPlugin::remove_scene_format_importer_plugin(const Ref<EditorSceneFormatImporter> &p_importer) {
	ERR_FAIL_COND(!p_importer.is_valid());
	ResourceImporterScene::remove_importer(p_importer);
}
void EditorPlugin::add_scene_post_import_plugin(const Ref<EditorScenePostImportPlugin> &p_plugin, bool p_first_priority) {
	ResourceImporterScene::add_post_importer_plugin(p_plugin, p_first_priority);
}
void EditorPlugin::remove_scene_post_import_plugin(const Ref<EditorScenePostImportPlugin> &p_plugin) {