[BUGFIX] Fluid tries to render 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 instead.

Releases: master, 6.2, 6.1
Resolves: #47188
Change-Id: I9c168f4793f173f6c2f848e6e3da1f6021ae9c06
Reviewed-on: http://review.typo3.org/23951
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>