[TASK] Improve usability of EXT:impexp

Improve usability at:
- Use public url instead of combined identifier to show the upload folder
- Don't show a warning if the empty value is selected of extension dependencies

Change-Id: I365d0a3192393a25ebbb85af27df9246c677fba8
Resolves: #72695
Releases: master, 7.6
Reviewed-on: https://review.typo3.org/45879


Reviewed-by: Wolfgang Klinger <wolfgang@wazum.com>
Reviewed-by: Bernhard Kraft <kraftb@think-open.at>
Tested-by: Bernhard Kraft <kraftb@think-open.at>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>