Reduce usage of `import cudf` internally

Came up during https://github.com/rapidsai/cudf/pull/10791, we should work towards an architecture where we don't need to fully import the top level `cudf` namespace in internal areas of the code. Doing so creates a state that is a little prone to circular import issues especially in python and goes against the general onion model of dependencies.

We do this kind of [everywhere](https://github.com/rapidsai/cudf/search?q=%22import+cudf%22) so I'd suggest doing this kind of from the inside out starting with the most base level classes and then working towards dataframe and series. If we find it's not possible to decouple things at some point we can revisit.