Clean up public store API

[dstansby]

This cleans up the public API surface for stores, by making all the zarr.store submodules private, and exposing the API in the zarr.store namespace. I think this is what was always intended, but changing the sub-module names to start with a _ makes sure they're not documented.

Tagged as 3.0, because this is an API change it would be good to get in before final release.

Fixes #2602

[jhamman]

What do we think about also making the following objects private:

• StoreLike
• StorePath
• WrapperStore
• make_store_path

We can always make them public later but these four seem like good candidates to keep private for now.

[dstansby]

StoreLike and StorePath should part of the public API, because they're types of arguments of public functions (e.g., create(), from_dict()), WrapperStore should be public because LoggingStore inherits from it.

I guess make_store_path could be private, but I think out of scope for this PR to give that potential change more visibility to other devs instead of hiding it here.

[jhamman]

I opened #2628 for the make_store_path issue which can go in after this.