`QuerySet._extract_model_params` can/should be enchanced to support lazy defaults

[Viktor Danyliuk]

I wish to have ability to write something like this:

from django.utils.functional import lazy

obj, created = model.objects.get_or_create(
     key=jwt,
     defaults=lazy(self.get_defaults_for_model, dict)(jwt) 
)

But at the moment _extract_model_params prepare defaults before it's realy needed.
I think _extract_model_params should be separated to _prepare_model_lookup and _prepare_model_params.
So it will be possible to call _prepare_model_params when model.DoesNotExist or even inside _create_object_from_params.