[TASK] Deprecate TYPO3_mainDir

This patch deprecates the constant "TYPO3_mainDir"
in favor of a BackendEntryPointResolver which
contains the information in a unified way to work with
the prefix for the path or URL.

Some tests are adapted properly to run in "frontend
context" now (e.g. felogin") to simulate the proper
behavior.

Resolves: #98283
Releases: main
Change-Id: I8d7c410456a9fe41d220aa481032df251e949936
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75620


Tested-by: Susanne Moog <look@susi.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <bfr@qbus.de>
Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Benjamin Franzke <bfr@qbus.de>