Remove (unused) ValueMap from API, simplify and unit test RepositoryMapImpl

The value map seems to offer little value and indeed is not used in the repository itself.

The repository map has a lot of intricate logic for hierarchy resolution and query based configuration. AFAICS this logic is not used. It is certainly not subject to unit tests. It also does not respect the basic Map contract. (the #entrySet does not contain properties, while they are resolved by #get, for instance)

My proposal is to simplify the repository map implementation, such that it does implement a (read-only) Map.

Let's deprecate the ValueMap and remove it in a later release.