avoid making gathered facts high precedence, only set_fact is supposed to be.
vars set via set_fact with cacheable are higher precedence than plain facts.
Previously (after 6fbd0a8bb5) regular facts would end up with a
higher precedence than host or play vars, but they should not be. Facts were getting added to 'non_persistent_facts' (equivalent to 'register' vars) which is higher precedence than facts should be.
added 'cacheable set_facts' to precedence docs
'ansible_facts_cacheable' -> '_ansible_facts_cacheable' (made 'private')
41685fb516