When specifying --templates do not use internal templates

[thirtytwobits]

If --templates is provided the current implementation will silently fall back to built-in templates if the user-supplied ones are not found. We should disable all use of internal templates when the user provides this override but add an optional --fallback-to-builtin switch to allow the user to mix-and-match. This avoids "magical" behavior that is hard to diagnose.

[thirtytwobits]

This changed awhile back with the introduction of ResourceSearchPolicy such that the title of this feature was met. --fallback-to-builtin-templates will be added in 3.0 to finish this out.