EZP-22465: Decouple build of handlers in Legacy storage engine

This PR resolves issue https://jira.ez.no/browse/EZP-22465

These are first 3 commits extracted from https://github.com/ezsystems/ezpublish-kernel/pull/792 for separate review.

Circular dependency between ContentHandler and ContentTypeHandler is removed by introducing internal TreeHandler as an intersect between the two.
Circular dependency between FieldHandler and ContentType handler is removed by loading and passing ContentType to it from ContentHandler.
Deprecated ContentMapper dependency is removed from Location Handler.