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 <redacted>
---
 http-push.c   |  6 +++---
 http-walker.c | 26 +++++++++-----------------
 http.c        | 21 ++++++++-------------
 http.h        |  5 +++--
 packfile.c    |  9 +++++++++
 packfile.h    |  8 ++++++++
 6 files changed, 40 insertions(+), 35 deletions(-)

[snip]

diff --git a/packfile.c b/packfile.c
index 4d2d3b674f3..6aa2ca8ac9e 100644
--- a/packfile.c
+++ b/packfile.c

@@ -121,6 +121,15 @@ void packfile_list_append(struct packfile_list *list, struct packed_git *pack)
 	}
 }
 
+struct packed_git *packfile_list_find_oid(struct packfile_list_entry *packs,
+					  const struct object_id *oid)
+{