[!!!][TASK] Remove support for non namespaced classes in Extbase

Non-namespaced classes will no longer be respected in Extbase.
This affects all places where Extbase magic happens, like translating
the model name to repository name (and vice versa).

If you still use class names with underscores, especially models,
repositories and controllers you need to act now.

Releases: master
Resolves: #85036
Change-Id: Ide5923159027802e723dff49729bba52a74c2639
Reviewed-on: https://review.typo3.org/56990


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>