[BUGFIX] Make indexed search extbase plugin work after class alias change

Indexed search extbase experimental plugin (pi2) requires class alias
to work. This patch moves it back from compatibility6 extension
to indexed_search.

Resolves: #64886
Releases: master
Change-Id: I36d288d80453a0772181358222608e1df83f9cf6
Reviewed-on: http://review.typo3.org/36681


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>