abstraction leakage in codec registration #3339

the codec classes are added to the registry in the same module where they are defined, e.g.

from zarr.registry import register_codec


class FooCodec():
   ...

register_codec("foo", FooCodec)

This is problematic because it prevents the codec registration / resolution logic from importing anything from the codecs, because this will cause circular imports. I think we can use the organization of the data type registry as a model for improving this.