[BUGFIX] Add conflicts to core extensions' composer.json

Core extensions installed individually (from subtree split)
must never be installed together with the typo3/cms package

We can avoid this, by adding a conflict section in each
composer.json

Resolves: #81982
Releases: master, 8.7
Change-Id: Ia6a346227852237f58875a3e3216d3cad298f8bf
Reviewed-on: https://review.typo3.org/53603


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Sascha Egerer <sascha@sascha-egerer.de>
Reviewed-by: Helmut Hummel <typo3@helhum.io>
Tested-by: Helmut Hummel <typo3@helhum.io>