packfile: inline `find_kept_pack_entry()`

packfile: inline `find_kept_pack_entry()`

The `find_kept_pack_entry()` function is only used in
`has_object_kept_pack()`, which is only a trivial wrapper itself. Inline
the latter into the former.

Furthermore, reorder the code so that we can drop the declaration of the
function in "packfile.h". This allows us to make the function file-local.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>