[BUGFIX] Fix call to undefined function when importing t3x

When importing a t3x file that contains files the function
"getFiles" of the object TYPO3\CMS\Core\Resource\Folder is called
which does not exist.

Resolves: #60474
Releases: 6.3, 6.2
Change-Id: Ibc392bced57a85ad6eae333406ff507af7faefd4
Reviewed-on: http://review.typo3.org/31744
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>