Upgrade Wizard "Install Outsourced System Extensions" should only show not-installed extensions
The Upgrade Wizard "Install Outsourced System Extensions" (tx_coreupdates_installsysexts) suggests the user to install all system extensions, even those which are already installed. This is confusing to the user that is doing a "new installation" based on the intro package for example, where all those extensions are already installed by default.
Solution would be to do the same logic as we have in "tx_coreupdates_installnewsysexts", which checks every extension if they are installed (and if all are installed, don't present the wizard at all!).
(issue imported from #M17429)