isFile() was used in sense of !isDirectoy.
isDirectoy(), exists() and lastModifiedTime() can be fetched at once
from OS using Files.readAttributes()

Change-Id: I482b1c4ab195684e3de221f68ea7d104a66b4a9e
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/185824
Tested-by: PDE Bot <pde-bot@eclipse.org>
Reviewed-by: Julian Honnen <julian.honnen@vector.com>