[TASK] Cleanup ext:lowlevel

* Fix path to commands in ext_localconf.php
* Remove require_once calls
* Move code from dbint/index.php to constructor
* Move code from config/index.php to constructor

Change-Id: I288834f66427c2a11836c87b973c92f9621c9c3f
Resolves: #49552
Releases: 6.2
Reviewed-on: https://review.typo3.org/21774
Reviewed-by: Alexander Opitz
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring