clean superfile and cachefile paths upon creation

otherwise, if the output dir is the root, the path would be denormalized.

the code for finding existing files already does that.

Change-Id: I56d70477e9c9ffcd936325068624a84df10ffd87
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
(cherry picked from qtbase/72075656cfeac570ce002c517a22b1c5d7bc0dd0)
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>