midx: rename pack_info to write_midx_context

midx: rename pack_info to write_midx_context

In an effort to streamline our chunk-based file formats, align some of
the code structure in write_midx_internal() to be similar to the
patterns in write_commit_graph_file().

Specifically, let's create a "struct write_midx_context" that can be
used as a data parameter to abstract function types.

This change only renames "struct pack_info" to "struct
write_midx_context" and the names of instances from "packs" to "ctx". In
future changes, we will expand the data inside "struct
write_midx_context" and align our chunk-writing method with the
chunk-format API.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>