midx: expose `midx_layer_contains_pack()`

midx: expose `midx_layer_contains_pack()`

Rename the function `midx_contains_pack_1()` to instead be called
`midx_layer_contains_pack()` and make it accessible. Unlike
`midx_contains_pack()` (which recurses through the entire chain), this
function checks only a single MIDX layer.

This will be used by a subsequent commit to determine whether a given
pack belongs to the tip MIDX layer specifically, rather than to any
layer in the chain.

No functional changes are present in this commit.

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