[TASK] Refactor GU::getFileAbsFileName

Improve readability of the method by extracting distinct cases into own
blocks with early return statements.

Case 1: "EXT:" path
Case 2: (already) absolute path
Case 3: relative public path

Resolves: #100094
Releases: main
Change-Id: I664c3611ef86c779c0398c7a4f938297eb591a81
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/78034


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>