[BUGFIX] Fix impexp for importing exports from version 4.x

This patch introduces FAL support in impexp for the import of
exports done in version 4.x by using a legacy mode. Related images
from uploads/ (tt_content:images and pages:media) are moved to
the folder /_imported/ below users default storage.
Linked files and RTE images are moved to the original folder
structure below the folder /_imported/ below users default storage.

Also the images additional information, like titleTexts and links are
migrated to the new created file reference record.

These scenarios are covered with functional tests:
* pages media
* pages_language_overlay media
* tt_content images
* links to files
* links to files in RTE
* images in RTE
* other records using fields of type "group" internal_type "file"
  or "file_reference" in TCA and FlexForm fields.

Resolves: #52593
Releases: 6.2
Change-Id: I170ec20827af255f85ddc0fd51ffe30df7ef6b4e
Reviewed-on: https://review.typo3.org/22939
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert