Refactor `DeclaredQuery` to decouple the query definition from its introspected state

[mp911de]

Follow-up to #2989.

We should decouple DeclaredQuery properties (query string and native query flavor) from introspection properties (alias, usage of constructor expressions, …) to separate the process from DeclaredQuery creation and introspection so that QueryEnhancerFactory can be provided externally.