"Normal if used" processing role

[robander]

Creating an issue out of a card that was on the DITA 2.0 project page but without additional detail.

The original request from Eliot was to create a "normal if used" value for @processing-role; the idea is that something can be treated as resource-only unless otherwise referenced in the context of the root map; the presence of that reference would cause it to be treated as normal.