[BUGFIX] Ensure cache clearing is done properly by EM

This fixes problems with access to classes from ext_localconf
files that were discarded from the classes cache at the wrong
time.

Resolves: #54906
Releases: 6.2
Change-Id: Icf650261a5ed72c36d332e3407ababf627968ab2
Reviewed-on: https://review.typo3.org/28372
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Thomas Maroschik
Tested-by: Thomas Maroschik
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny