You must set the environment variable MW_INSTALL_PATH to your mediawiki root to use this extension.
In order to install on a Percona DB this extension tells composer how to autoload it. This is necessary so that when you specify “percona” as the db type during installation, it can find the right db driver. It also allows MediaWiki to use $wgDBtype to find the provided DatabasePercona class.
Because of this bootstrapping, composer’s autoloader needs to be able to use MediaWiki’s autoloader and we make it available by loading $MW_INSTALL_PATH/includes/AutoLoader.php.