[BUGFIX] Use indexed menu item array for directory menu

This prevents overwriting existing entries when a mount point
and its target have the same parent.

Resolves: #89402
Releases: master, 9.5
Change-Id: I1e527443410e0f875a95e803d4c3c026b089f5a7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61962


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Susanne Moog <look@susi.dev>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Susanne Moog <look@susi.dev>