[v3] reorganize package - move most modules to zarr.core

[jhamman]

This PR creates a core subpackage in the zarr package and begins to move some modules there. As I explained in #2037 (comment), this is designed to clarify which parts of the zarr project are public API and which are internal (everything in core).

In my in my initial commit, I only moved array.py, config.py, group.py, indexing.py, and sync.py. More to be done but I wanted to put this up for an initial reactions before going through the complete set of modules.

closes#2037

[normanrz]

Lgtm

[dstansby]

As a general comment, objects should only be listed in one all throughout the codebase. This should be where we are expecting users to import the object from. We are still free to import from other locations internally, but all signals where the public API lives.

[jhamman]

@zarr-developers/python-core-devs - this is ready for a real review now. Thanks @dstansby and @normanrz for an initial look. With these changes, the package organization now looks like this:

.
├── __init__.py
├── abc
│ ├── __init__.py
│ ├── codec.py
│ ├── metadata.py
│ └── store.py
├── api
│ ├── __init__.py
│ ├── asynchronous.py
│ └── synchronous.py
├── codecs
│ ├── __init__.py
│ ├── _v2.py
│ ├── blosc.py
│ ├── bytes.py
│ ├── crc32c_.py
│ ├── gzip.py
│ ├── pipeline.py
│ ├── registry.py
│ ├── sharding.py
│ ├── transpose.py
│ └── zstd.py
├── convenience.py
├── core
│ ├── __init__.py
│ ├── array.py
│ ├── array_spec.py
│ ├── attributes.py
│ ├── buffer.py
│ ├── chunk_grids.py
│ ├── chunk_key_encodings.py
│ ├── common.py
│ ├── config.py
│ ├── group.py
│ ├── indexing.py
│ ├── metadata.py
│ └── sync.py
├── creation.py
├── errors.py
├── py.typed
├── registry.py
├── store
│ ├── __init__.py
│ ├── _utils.py
│ ├── common.py
│ ├── local.py
│ ├── memory.py
│ └── remote.py
├── strategies.py
├── testing
│ ├── __init__.py
│ ├── buffer.py
│ ├── store.py
│ └── utils.py

[dstansby]

LGTM! I left some optional suggestions to move the __all__ to the tops of files.

Once this is in it should unblock #2002, which will make it easier to see what the documented public API looks like 😄

[d-v-b]

I like it! I do think we should move the __all__ declarations to the top of each file as per @dstansby's suggestion, but other than that I think it's good to go

[d-v-b]

merging! thanks @jhamman