[TASK] Move ExtDirect component registration to EXT:backend

Although this functionality will be removed anyway as soon as possible,
moving this code to EXT:backend where it belongs cleans up the general
bootstrap class and separates the logic into the system extension that
is referenced.

Resolves: #81060
Releases: master
Change-Id: I1e1a4d2a791e17a52c8298e08299a3ee74ee37dc
Reviewed-on: https://review.typo3.org/52649


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Thomas Hohn <thomas@hohn.dk>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>