@barry 

> Interdependent namespace packages.  If they get loaded in the wrong order, they can mess up __path__ settings

Actually, when writing the PR, I had a revelation how this could be implemented. Via an import hook that would work like a union FS!

In its .pth file, each such package will import the hook's module (which will cause the hook to be installed on the first import) and "register" its namespaces and/or dependencies with it. The hook will then calculate the required load order and enforce it upon import of any of the registered namespaces.