ModuleExists performance bottle neck

the moduleExists() method is called excessively, and there's no caching associated with it. In one profile run to import 2000 records, this query was executed around 17,000 times. When the mysql connection is remote, this is a heavy bottleneck and accounted for 15% of the transaction time.

This should be optimized to at least cache the result of the check, since it's not going to change between row imports.