(Don't Merge)(FACT-753) Update default external fact directories for unified agent layout

This commit updates the default search directory for external facts.
We now look in /opt/puppetlabs/agent/facts.d for both open source and
PE versions of Facter.

This change is part of the unified agent effort.
@kylog Ah yeah, that sounds totally reasonable. I'm heading away for the day but I'll update to include all three directories, old and new, tomorrow.