[!!!][TASK] EM: Drop support for extensions-in-extensions installation

A feature which was built for the introduction package back then, which
allowed to ship a modified / custom extension as dependency, where
one could put another extension in EXT:my_ext/Initialisation/Extensions/another_ext
is removed, as this special case is

a) not needed anymore
b) not following any rules of being conformant for "dependency hell"
c) specific for dependency handling of TYPO3 (non-composer)

Resolves: #92532
Releases: master
Change-Id: I7e7d8681a81e184835122ae221971e5eab435778
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66097


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>