ok, as far as I understand
...is meant to be...
include_once( __DIR__ . '/vendor/autoload.php' );
require_once( __DIR__ . '/../vendor/autoload.php' );
...and autoload.php will resist in wikiroot/vendor together with additional
packages which are not mediawiki-extensions.
We do not need multiple autoloader.php.
Well for example if I want to install an extension locally from disk
(zip-archive) into directory extensions I have to change