[TASK] Change relevance sorting in EM

The Extension Manager creates a virtual "position" field containing
the relevance of a search hit. The relevance is calculated in a hacky
way working in MySQL only. This is replaced by a more understandable
query that is parsable by the SqlParser.

Resolves: #61204
Releases: 6.3, 6.2
Change-Id: I541cbeef745e8dc25131af2a937ec2d028a2fc2c
Reviewed-on: http://review.typo3.org/32796
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>