In 2018, we moved most of Parameterized onto a param namespace object, expecting this to be the public API of param 2.0, and cleaning up the namespace of user classes.

The new API has been in use for a while within holoviz projects, but we're still changing it. Meanwhile, the previous API remains available.

The original API's tests were copied into an API0 subdirectory, while tests in API1 use the new API.

(Probably not ideal to just copy everything, and cleaning up would be great, but this explains the two directories you see here.)