Refactor CheckoutCheckpoint into separate experiment and checkpoint functions

[bfirsh]

CheckoutCheckpoint on the Project Go struct current serves two purposes:

• Check out a checkpoint for Python API
• Check out an experiment or checkpoint for CLI

There should be separate methods for checking out projects and checkpoints on Project. The business logic of choosing between an experiment and checkpoint, along with all the log output, should live in the CLI or somewhere else.

See also:

• https://github.com/replicate/replicate/pull/408#discussion_r551683876
• https://github.com/replicate/replicate/pull/467#discussion_r553652600

Follow-on of #408