[BUGFIX] impext: export is broken

Some uses of the LocalPageTree class do not use the namespaced version.
Use the correct class name.

Fixes: #43317
Releases: 6.0

Change-Id: I46a9fd4ca4ad920b430738587dca9c47b901e033
Reviewed-on: http://review.typo3.org/16760
Reviewed-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel