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

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.