QueryBuilder - replace deprecated getAlias() by getAliases()[0]

a QueryBuilder is not guaranteed to have a single root alias. You can have several ones. So for external code using the API, they should be aware of this and use ``getRootAliases`` (the method ``getRootAlias`` comes from a mistake in the 2.0 release where the public API of the QueryBuilder forgot about the fact there can be multiple root aliases IIRC)