Always check Composer extensions independantly of ExtensionRegistration
Sometimes, when MediaWikiFarm is in monoversion mode, ExtensionRegistry
is known, and we should check if some extension should be loaded with
Composer; it was no more the case since ca185e0.
Also, the function MediaWikiFarmConfiguration::detectLoadingMechanism
no more tries to detect Composer extension given the case is handled
in an other way.