http: refactor subsystem to use `packfile_list`s

http: refactor subsystem to use `packfile_list`s

The dumb HTTP protocol directly fetches packfiles from the remote server
and temporarily stores them in a list of packfiles. Those packfiles are
not yet added to the repository's packfile store until we finalize the
whole fetch.

Refactor the code to instead use a `struct packfile_list` to store those
packs. This prepares us for a subsequent change where the `->next`
pointer of `struct packed_git` will go away.

Note that this refactoring creates some temporary duplication of code,
as we now have both `packfile_list_find_oid()` and `find_oid_pack()`.
The latter function will be removed in a subsequent commit though.

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