Add relative path support for `EditorPlugin::add_autoload_singleton`

editor/editor_plugin.cpp

@@ -62,7 +62,14 @@ void EditorPlugin::remove_custom_type(const String &p_type) {
 }
 
 void EditorPlugin::add_autoload_singleton(const String &p_name, const String &p_path) {
-	EditorNode::get_singleton()->get_project_settings()->get_autoload_settings()->autoload_add(p_name, p_path);
+	if (p_path.begins_with("res://")) {
+		EditorNode::get_singleton()->get_project_settings()->get_autoload_settings()->autoload_add(p_name, p_path);
+	} else {
+		const Ref<Script> plugin_script = static_cast<Ref<Script>>(get_script());
+		ERR_FAIL_COND(plugin_script.is_null());
+		const String script_base_path = plugin_script->get_path().get_base_dir();
+		EditorNode::get_singleton()->get_project_settings()->get_autoload_settings()->autoload_add(p_name, script_base_path.path_join(p_path));
+	}
 }
 
 void EditorPlugin::remove_autoload_singleton(const String &p_name) {