repack: remove 'existing_packs' API from the builtin

repack: remove 'existing_packs' API from the builtin

The repack builtin defines an API for keeping track of which packs
were found in the repository at the beginning of the repack operation.
This is used to classify what state a pack was in (kept, non-kept, or
cruft), and is also used to mark which packs to delete (or keep) at the
end of a repack operation.

Now that the prerequisite refactoring is complete, this API is isolated
enough that it can be moved out to repack.[ch] and removed from the
builtin entirely.

As a result, some of its functions become static within repack.c,
cleaning up the visible API.

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