Fix css loading in plugins (#12573)

- allow css loader to be imported again (wasnt prefixed by plugin)

public/app/features/plugins/plugin_loader.ts

@@ -56,7 +56,7 @@ System.config({
     css: 'vendor/plugin-css/css.js',
   },
   meta: {
-    'plugin*': {
+    '*': {
       esModule: true,
       authorization: true,
       loader: 'plugin-loader',