files_pack_refs(): use reference iteration

files_pack_refs(): use reference iteration

Use reference iteration rather than `do_for_each_entry_in_dir()` in
the definition of `files_pack_refs()`. This makes the code shorter and
easier to follow, because the logic can be inline rather than spread
between the main function and a callback function, and it removes the
need to use `pack_refs_cb_data` to preserve intermediate state.

This removes the last callers of `entry_resolves_to_object()` and
`get_loose_ref_dir()`, so delete those functions.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>