[FEATURE] Allow the activation of packages during runtime

Due to the nature of the Flow Package Manager, packages cannot
be activated and directly used during runtime. Before the Package
Manager it was possible to activate/deactivate extensions in
AdditionalConfiguration.php under certain custom conditions.

This patch introduces a new setting in $GLOBALS['TYPO3_CONF_VARS']
['EXT']['runtimeActivatedPackages'] = array('{packageKey}') that
gets initialized right after the package management initialization.

Resolves: #53015
Releases: 6.2
Change-Id: Id3b85a3feb00876d2a04a02e85450a4568eb5bff
Reviewed-on: https://review.typo3.org/24939
Reviewed-by: Thomas Maroschik
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Tested-by: Alexander Stehlik
Tested-by: Frans Saris
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Markus Klein
Tested-by: Markus Klein