[BUGFIX] Don't link extension root package to vendor folder

In case extensions are installed in vendor (in a future
version of composer installers), linking an extension root
package to the target location (vendor) does not make sense
and is now skipped.

Resolves: #96021
Releases: master
Change-Id: Ifdee432c10135df35f3a47e54d2c366714ca32ba
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72223


Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Benni Mack <benni@typo3.org>