`Cubical.**.Foo` should only import `**.Foo.Base` and `**.Foo.Properties`

[Saizan]

I think a Foo module should export what you're more likely to need about Foo.

If they re-export everything that's been developed for Foo then they are not as useful to import, as that slows down typechecking from scratch and bring a lot of names into the namespace.

We should also add this policy to CONTRIBUTING.md.