[BUGFIX] Fix relative path resolution in include static

Fix the usage of relative paths in INCLUDE_TYPOSCRIPT inclusions of
static templates from extensions.

EXT:myext/Configuration/TypoScript/setup.txt:
<INCLUDE_TYPOSCRIPT: source="DIR:./Setup/">

EXT:myext/Configuration/TypoScript/Setup/ holds some TS files.

Resolves: #57447
Releases: 6.2
Change-Id: I64ba190fa6959eb27a2d6f1c278cfb9c9c3cbfaf
Reviewed-on: https://review.typo3.org/29468
Reviewed-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Jan Kiesewetter
Tested-by: Jan Kiesewetter
Reviewed-by: Markus Klein
Tested-by: Markus Klein