[TASK] Cleanup __DIR__ and require/require_once

* Use __DIR__ instead of dirname(__FILE__) (since PHP 5.3.0)
* Use require_once for loading classes ("just once")

Resolves: #47203
Releases: 6.2
Change-Id: I4b5d003a0bbabd91dcc7e58081eed41b5314da3f
Reviewed-on: https://review.typo3.org/19952
Reviewed-by: Xavier Perseguers
Reviewed-by: Oliver Klee
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Pascal Dürsteler
Reviewed-by: Stefan Froemken
Tested-by: Stefan Froemken
Reviewed-by: Markus Klein
Tested-by: Markus Klein