(CFACT-151) Add path fact

Since there's a single implementation for all platforms, could/should we add this in `facter::facts::collection::add_default_facts` (and then moot the change to each of the different platform-specific `add_platform_facts` impls)?

I might be misremembering, but I thought we had an `add_common_facts` in that class at one point, intended for completely common facts/resolvers.