Added support for pluginfactsource, required for SRV and 3.7

Needed for supporting SRV and 3.7. More at https://tickets.puppetlabs.com/si/jira.issueviews:issue-html/PUP-1035/PUP-1035.html
Not sure I like having a different default than the current stable puppet, but it is more future proof. @domcleal should we just go with this and bump the module on release?