Facter should load its config file (excluding the CLI section) when it is used as a library.
If the library consumer specifies any configuration, that should take precedence (similar to how command line overrides facter.conf in standalone mode)
The Facter config file will now be loaded from its default location (/etc/puppetlabs/facter/facter.conf on Unix, C:\ProgramData\PuppetLabs\facter/facter.conf on Windows) when Facter is required from Ruby. Two settings will be respected: external-dir and custom-dir, which add paths to search for custom and external facts to those already specified by Puppet.