HSEARCH-1724 Simplify access to ClassLoaderService

This is needed for the boot refactoring, the SearchFactory uncoupling from its SPI, and also avoids some map lookups during Query execution.

 - https://hibernate.atlassian.net/browse/HSEARCH-1724
what if another service is needed as well? Probably unlikely, but `ServiceManager` seems more generic at this point imo.