Clobbered class names in ClassSelector.get_range

We found out that classes with the same name (e.g. in different files) get clobbered in `concrete_descendents`. We extended `descendents` with a `concrete` parameter, and added a warning in `concrete_descendents` (https://github.com/holoviz/param/pull/1035/).

We have a problem with `ClassSelector.get_range`, where classes with the same name are still clobbered. Options include:
- Updating somehow the returned type to a list of tuples
- Creating another method like range that doesn't suffer from this problem
- Returning a MultiDict
- etc.

A couple of difficulties with `get_range`:
- It's implemented by other `Selector` subclasses, so I guess their return type should be identical, or at least behave the same way
- It's used quite a bit internally by Panel at least (maybe more, to be checked)

So this needs a nice transition plan.