Rework NamespaceProvider and NamespacedEntity in M3

[enikao]

On 2023-07-07, we decided to remove NamespaceProvider from M3, and rename NamespacedEntity to IKeyed (#142).

Rationale: NamespaceProvider is not really needed. It was primarily meant to establish the semantic idea of a namespace. We still have the semantic idea, but don't need a manifestation in M3.
Calculating a qualified name can work by nested INamed elements. Once we have computed features (#6) we'll probably add a computed feature qualifiedName: String to INamed for this purpose.
Earlier, we thought about using fully qualified names to identify language entities. This is not relevant any more, as we have meta-pointers (#89).

Overall, this simplifies the M3 a lot.