### 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)
EditorNode::get_singleton()->remove_resource_conversion_plugin(p_plugin);
}
void EditorPlugin::_editor_project_settings_changed() {
	emit_signal(SNAME("project_settings_changed"));
}
void EditorPlugin::_notification(int p_what) {
	switch (p_what) {
		case NOTIFICATION_ENTER_TREE: {
			EditorNode::get_singleton()->connect("project_settings_changed", callable_mp(this, &EditorPlugin::_editor_project_settings_changed));
		} break;
		case NOTIFICATION_EXIT_TREE: {
			EditorNode::get_singleton()->disconnect("project_settings_changed", callable_mp(this, &EditorPlugin::_editor_project_settings_changed));
		} break;
	}
}
void EditorPlugin::_bind_methods() {
	ClassDB::bind_method(D_METHOD("add_control_to_container", "container", "control"), &EditorPlugin::add_control_to_container);
	ClassDB::bind_method(D_METHOD("add_control_to_bottom_panel", "control", "title"), &EditorPlugin::add_control_to_bottom_panel);
	ClassDB::bind_method(D_METHOD("add_control_to_dock", "slot", "control"), &EditorPlugin::add_control_to_dock);
	ClassDB::bind_method(D_METHOD("remove_control_from_docks", "control"), &EditorPlugin::remove_control_from_docks);
	ClassDB::bind_method(D_METHOD("remove_control_from_bottom_panel", "control"), &EditorPlugin::remove_control_from_bottom_panel);
	ClassDB::bind_method(D_METHOD("remove_control_from_container", "container", "control"), &EditorPlugin::remove_control_from_container);
	ClassDB::bind_method(D_METHOD("add_tool_menu_item", "name", "callable"), &EditorPlugin::add_tool_menu_item);
	ClassDB::bind_method(D_METHOD("add_tool_submenu_item", "name", "submenu"), &EditorPlugin::add_tool_submenu_item);
	ClassDB::bind_method(D_METHOD("remove_tool_menu_item", "name"), &EditorPlugin::remove_tool_menu_item);
	ClassDB::bind_method(D_METHOD("get_export_as_menu"), &EditorPlugin::get_export_as_menu);
	ClassDB::bind_method(D_METHOD("add_custom_type", "type", "base", "script", "icon"), &EditorPlugin::add_custom_type);
	ClassDB::bind_method(D_METHOD("remove_custom_type", "type"), &EditorPlugin::remove_custom_type);