[BUGFIX] Reload available packages information

If a new extension is fetched or copied to typo3conf directory, the list
of available extensions has to be reloaded.

Resolves: #71455
Releases: master
Change-Id: I512fa2a404271bc46e616b2ae4458798cf73cd8d
Reviewed-on: https://review.typo3.org/44657


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>