[BUGFIX] Don't use folders as templates, partials and layouts

The file_exists checks in TemplateView and StandaloneView are true for
folders. This leads to unrendered partials with resorted paths.
Use is_file.

Releases: 6.1, 6.0, 4.7, 1.3
Fixes: #47186
Fluid Package Issue: #47188

Change-Id: Ib64e180e01d66777bb5b0683cce26d88ec2a9fd6
Reviewed-on: https://review.typo3.org/19920
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters