[BUGFIX] Make Import/Export work with Package Manager

When displaying the import/export backend module
there are warnings and an empty extension list,
because the extension Selector does not use the
API to fetch the extension list, but does a raw
$TYPO3_LOADED_EXT instead.

Resolves: #56670
Releases: 6.2
Change-Id: I851cc1f7e01acd9f743559a9400340364280ba7e
Reviewed-on: https://review.typo3.org/28183
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring