[TASK] Improve composer repository setup

In order to test local composer packages, they need to be placed under
the "packages" directory in "Build/composer". To autoload these packages
automatically, the necessary setup is adjusted.

Resolves: #102054
Releases: main, 12.4
Change-Id: I6c40a06142efaf13493a149efc636da9837c3598
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81268


Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>