[BUGFIX] Fix some minor Composer mode path inconsistencies

* Use config API instead of env vars when building package artifact
* Don't try to scan for extensions in Composer mode
* Don't create extension directory in Composer mode, as
  the composer installer takes care of creating it if needed
* Don't try to scan extension directory if it does not exist
* Use package type instead of install path to distinguish core extensions

Releases: master
Resolves: #95483
Change-Id: I63ff42c301eba66e85471702fb03739374f7821c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71453


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Helmut Hummel <typo3@helhum.io>