Thread:Project:Support desk/MediaWiki extensions loaded by Composer

We've switched from using Semantic Bundle to install extensions by using Composer. Now we can skip, for some extensions, manually including entry points at the LocalSettings.php file (http://www.bn2vs.com/blog/2013/11/24/introduction-to-composer-for-mediawiki-developers/). But how does MediaWiki know which extensions to load. Does it read composer.json?