[TASK] Decouple TemplateService->getFileName

TemplateService in TYPO3 frontend is responsible for
fetching sys_template records and sorting TypoScript
related things out, but also contains one method,
which is nowadays completely separate from
sys_template: "getFileName".

This method checks for valid syntax and returns
a proper string then (resolves "EXT:... syntax"
and valdiates against available paths).

The logic is frontend-related, thus moved
to EXT:frontend/FilePathSanitizer.

The old functionality in TemplateService is
deprecated, along with the public property "fileCache".

Resolves: #85445
Releases: master
Change-Id: Id52ef476d0cd6a67de2560a0e427339b7310427e
Reviewed-on: https://review.typo3.org/57403


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>