packfile: use higher-level interface to implement `has_object_pack()`

packfile: use higher-level interface to implement `has_object_pack()`

In `has_object_pack()` we're checking whether a specific object exists
as part of a packfile. This is done by calling the low-level function
`find_pack_entry()`, but this function will eventually be moved into
"odb/source-packed.c" and made file-local.

Refactor the code to use `packfile_store_read_object_info()` instead.
This refactoring is functionally equivalent as that function will call
`find_pack_entry()` itself and then return immediately when it ain't got
no object info pointer as parameter.

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