packed_ref_iterator_begin(): iterate using `mmapped_ref_iterator`

packed_ref_iterator_begin(): iterate using `mmapped_ref_iterator`

Now that we have an efficient way to iterate, in order, over the
mmapped contents of the `packed-refs` file, we can use that directly
to implement reference iteration for the `packed_ref_store`, rather
than iterating over the `ref_cache`. This is the next step towards
getting rid of the `ref_cache` entirely.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>