poc: singular pypi repo for late binding external dependencies

[rickeylev]

POC for having a single @pypi repo so that targets can depend on something without
having to choose a particular dependency closure (pip.parse hub_name).

The current impl works in a hub-of-hubs fashion, however, I think that indirection
could be removed, since it's known at bzlmod time the spokes that a pip.parse hub points to.

[aignas]

I like this API, this is reasonably easy to understand and is probably easier than doing transitions or anything else that would resolve/override a particular dependency from the hub.