delta-find: move sparse-revlog delta checks in the associated class Lets move the specialized code in the specialized class.

delta-find: split the _DeltaSearch class in two We now have things sliced small enough to have two class that use different `_iter_groups` implementation to encode their different logic. The filtering code remains to be moved, but I would rather keep this changeset simple and move them in the next.

delta-find: finish reworking the snapshot logic and drop more layer The refining logic only applies to the snapshot logic, and this is now all contained in a dedicated method. Along the way, we drop the refined_groups // raw_groups layer as they no longer make sense. The result is a more explicit `iter_groups` method. This conclude the splitting and simplification of the groups generation. We are now ready to dispatch this in more diverse classes.

delta-find: move the base of the delta search in its own function That logic is complicated enough that is is worth puting in its own function. Another method will be introduced in the next changeset to deal with the actual refining.