Using Hiera hierarchy, we can provide a default value that makes sense
for the current operating system and still allow the user to provide
their own custom package name if necessary.

Makes the code simpler without changing the logic.

This directory is operating-system dependant.

These directory only make sense on some operating systems and should
already be part of base system or the syslog-ng package.  Do need to
manage these in the patterndb module.

According to hier(7), /var/cache/ is used to store miscellaneous cached
files, precisely what merged patterns not yet installed are.