Clean up zarr.core.buffer API surface

[dstansby]

This cleans up the zarr.core.buffer API, by making zarr.core.buffer._core private. Everything was already imported into zarr.core.buffer, but imports elsewhere in the library just needed changing.

[jhamman]

We should make sure we don't end up with two levels of underscores. My preference would be we do the full zarr._core switch rather than the sub-sub modules. Before moving forward with these, let's address the broader question of which parts of the zarr.core module need to be available as public API (#2621).

[jhamman]

@dstansby - do you have thoughts on how to proceed here given #2669?

I'd be happy to see this land today but I don't think it needs to block 3.0.0.

[dstansby]

Yeah, doesn't need to block 3.0.0, and needs updating, but would be good to keep it open to remind me to update it at some point