Change Doctype Instances Semantics

Currently the service checking the existence of document instances does not distinguish between index pages in the original space or in one of the delegate spaces. For the common use case where users want to determine whether or not the current space has information in form of a given doctype, it should make a difference.

Therefore the improved implementation should only check for a local index page and treat index pages in referenced spaces as not existent.

Although this change will break existing behavior it is not marked as such, as the current implementation is considered to be an issue to be corrected (aka bug).