builtin/repack.c: avoid dir traversal in `collect_pack_filenames()`

When repacking, the function `collect_pack_filenames()` is responsible
for collecting the set of existing packs in the repository, and
partitioning them into "kept" (if the pack has a ".keep" file or was
given via `--keep-pack`) and "nonkept" (otherwise) lists.

This function comes from the original C port of git-repack.sh from back
in a1bbc6c0176 (repack: rewrite the shell script in C, 2013-09-15),
where it first appears as `get_non_kept_pack_filenames()`. At the time,
the implementation was a fairly direct translation from the relevant
portion of git-repack.sh, which looped over the results of

    find "$PACKDIR" -type f -name '*.pack'

either ignoring the pack as kept, or adding it to the list of existing
packs.

So the choice to directly translate this function in terms of
`readdir()` in a1bbc6c0176 made sense. At the time, it was possible to
refine the C version in terms of packed_git structs, but was never done.

However, manually enumerating a repository's packs via `readdir()` is
confusing and error-prone. It leads to frustrating inconsistencies
between which packs Git considers to be part of a repository (i.e.,
could be found in the list of packs from `get_all_packs()`), and which
packs `collect_pack_filenames()` considers to meet the same criteria.

This bit us in 73320e49ad (builtin/repack.c: only collect fully-formed
packs, 2023-06-07), and again in the previous commit.

Prevent these issues from biting us in the future by implementing the
`collect_pack_filenames()` function by looping over an array of pointers
to `packed_git` structs, ensuring that we use the same criteria to
determine the set of available packs.

One gotcha here is that we have to ignore non-local packs, since the
original version of `collect_pack_filenames()` only looks at the local
pack directory to collect existing packs.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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