[TASK] Add external installed dependencies

For legacy mode installed version, the system ships self-installed packages for
`DeepL` and `Http\Discovery\`.

As composer-based installations ignore autoloading in `ext_emconf.php`, this can
be safely added to ensure external dependencies installed during release will
work out of the box.

ext_emconf.php

@@ -34,6 +34,8 @@
     'autoload' => [
         'psr-4' => [
             'WebVision\\Deepltranslate\\Core\\' => 'Classes',
+            'DeepL\\' => 'vendor/deeplcom/deepl-php/src',
+            'Http\\Discovery\\' => 'vendor/php-http/discovery/src',
         ],
     ],
 ];