Avoid spurious errors when searching more granular indexes

Avoid spurious errors when searching more granular indexes

The storage service automatically generates methods for each top-level search
class, like open-ils.storage.authority.keyword.see_from.controlled.atomic, but
here we were trying to access more granular methods (which storage does not
know about) when a granular index was used. This generated an ugly error in
the logs.

To avoid these errors, we'll only look for cross references from the broad
index class to which the granular index belonds.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@16720 dcc99617-32d9-48b4-a31d-7c20da2025e4