REF: move registry, Registry to dtypes.base
These are more closely related to ExtensionDtype than to our internal EADtypes.  More concretely, they are low-dependency, while dtypes.dtypes has some unfortunate circular dependencies.

I'd also like to move pandas_dtype and is_dtype_equal from dtypes.common so that those can be stricty "above" dtypes.dtypes in the dependency structure.  Saving that for another pass.