MDL-42044 installation Allow validation without enumerated directories

Some zip packing methods don't specifically enumerate directories, they
produce:
poodll/lang/en/filter_poodll.php

instead of:
poodll/
poodll/lang/
poodll/lang/en/
poodll/lang/en/filter_poodll.php

The unzipper still knows to produce the directories and is fine.