maint: more robust listing of valid faces.

We used to treat anything with a top level key in the faces hash as a valid
face; it makes more sense to filter that only to things that have at least one
implementation.

Previously we had to be super-careful not to accidentally touch the top level
for an invalid face, which set us up for future failure when someone wasn't
careful enough; now we can cope with that.

Paired-With: Max Martin <max@puppetlabs.com>