[TASK] Decouple SystemEnvironmentBuilder from Bootstrap

Do not rely on defined constants or methods from Bootstrap
to be usable on it own (later).
This is a preparatory refactoring to make the Application/Bootstrap
unaware of the entryPointLevel handling.

Releases: master
Resolves: #84099
Change-Id: Ia2b3b47062db87139e446c928faee75890ffec08
Reviewed-on: https://review.typo3.org/55963


Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>