FileUtils#getAllVersions(): respect the classifier!

Without this change, MiniMaven will gladly mistake an artifact with a
classifier as conflicting version of an artifact without a classifier.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Verify that FileUtils#getAllVersions() respects the classifier

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>