Refuse to load entities from not-included bundles

With the content entity datasource's loadMultiple() (and, thus, load()) method it's possible to load entities that are not actually included in the datasource (since their bundle is excluded from it). I think this might make problems if a new node of bundle A is created just as an admin disables bundle A for the datasource. In any case, it makes sense conceptually to only load/return items that are actually in the datasource.

I also added an @todo comment to that effect to ContentEntity::loadMultiple.