(#9546) Use $vardir/external_facts for storing external facts

Puppet loads local ruby facts from "$vardir/lib/facter#{File::PATH_SEPARATOR}$vardir/facts" so we do not want to store our external facts in this directory.  I could find no place where $vardir/facts is populated by plugin sync or any other mechanism so this location must be for facts that do not belong to a Puppet module and are installed locally on the destination system.
(#9546) Load external fact directories into facter config