Actually this is an interesting idea. To solve the naming problem I would propose to call such packages with an SE## suffix. Let‘s say we get count(where:). Then the package could be called PreviewSE220.
Two good parts about this approach are that we won‘t need to version lock the APIs if they don‘t depend on already version locked stdlib APIs, and last but not least we don‘t have to delete these packages after the APIs are merged into stdlib, a deprecation attribute should do the trick to notify the users.