[BUGFIX] Internal extension information is not updated properly

tx_em_Tools::refreshGlobalExtList() is triggered on installing
extensions. However the current implementation is wrong (early
return) and does not consider $TYPO3_CONF_VARS.

This misbehavior also prevents the Introduction Package to
correctly install accordant cf_* tables of workspaces and
extbase, since the information in $TYPO3_CONF_VARS is not
available globally.

Change-Id: I3a1b8ea9d2da40a52d9838ac8b1fe9551b6dfed8
Fixes: #31246
Reviewed-on: http://review.typo3.org/6270
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert