[BUGFIX] PackageStates is not updated in EM

Introduces a signal in the EM that emits right before extensions
will be installed. The package manager reacts on this signal and
scans the extension folders for new extensions.

Fixes: #52766
Releases: 6.2
Change-Id: Ida1146d98b2c7f280a079c7ce6737111186c95ac
Reviewed-on: https://review.typo3.org/24714
Reviewed-by: Sascha Egerer
Tested-by: Nicole Cordes
Reviewed-by: Markus Klein
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind