refactor ad-hoc entity class sourcing

[rkoschmitzky]

Goal

Get rid of sourcing entity classes like this getattr(importlib.import_module("..entities", __name__), entity_type.__name__).

Motivation

We have more and more use cases where we need to retrieve the actual entity class and this is an ugly approach.

Considerations

We could potentially expose a proper type map on the session instance that allows convenient access to entity classes.