Use Packs specification

[alexevanczuk]

This PR extracts the idea of packs into the packs gem, which is located here: https://github.com/rubyatscale/packs.

The idea of doing this is that the rest of the toolchain depends on the idea of packs, but currently does so through parse_packwerk. For example, rubocop-packs, pack_stats, use_packs, code_ownership, and more. Rather than rely on stimpack directly, which has rails adjacent dependencies which we want to avoid to keep the idea of packs relevant even without rails, extracting packs helps all the tools depend on a common, simple, rarely changing standard.

[technicalpickles]

Makes a lot of sense to me 👍🏻