[BUGFIX] Fix fatal errors upon package installation

During the installation of packages via the extension
manager a fatal error can occur when the ext_localconf
and ext_tables are loaded and use classes from their own
package. This happens because the package is activated
but the class loader is not aware yet of the new package.

This patch adds the extension temporarily to the runtime
activated packages in the class loader.

Fixes: #53795
Releases: 6.2
Change-Id: I195b86284b9a288f4d7c102168ec18ed8683cb07
Reviewed-on: https://review.typo3.org/27544
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel