FurnaceClasspathScanner#scan() and
FurnaceClasspathScanner#scanClasses() is almost the same -> DRY.

FurnaceClasspathScanner#handle() should go away.

FurnaceClasspathScanner#filenameToClassname() should go to windup-utils.

FurnaceScannerFilenameFilter could be replaced with a generic Filter<T>.

FurnaceScanner*Filter has nothing to do with Furnace, should be renamed.

FurnaceClasspathScanner should use Path instead of String.