[8.x] Fix autoresolving model name from factory

[driesvints]

At the moment, a factory can't auto-resolve namespaced models which are nested inside the models directory and have coresponding Factory classes at the same location. You can reproduce this with the issue at #40614

By changing the code here to take into account the nested namespace for factory classes, we can auto resort the model name without having to define the $model class on the factory.

One thing I'm not sure off is that if we should just fallback on the model classname without the namespace for the last part here (as it was before):

: $appNamespace.$factoryBasename;

I can adjust the code for that if you like.

Fixes #40614