repack: 'write_midx_included_packs' API from the builtin

repack: 'write_midx_included_packs' API from the builtin

Now that we have sufficiently cleaned up the write_midx_included_packs()
function, we can move it (along with the struct repack_write_midx_opts)
out of the builtin, and into the repack.h header.

Since this function (and the static ones that it depends on) are
MIDX-specific details of the repacking process, move them to the
repack-midx.c compilation unit instead of the general repack.c one.

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