Say where an association's reference is, and split out list-references' violations

[dduugg]

Summary

Two things left over from #71's review.

• schema/list-references-output.json now says that a reference pks infers from an association, as has_many :tacos gives ::Taco, is at the start of the call, where no constant is written. A class_name: option names the constant only in a string. query_packwerk's file cache looks for the constant's node in the file and finds none there. The description also says the source may write a constant relatively, as Bar inside module Foo.
• The loop that collects a reference's violations moves out of list into violations_of, which takes the nested loops out of list. The output is the same. The comment on the copies it merges now says that only the experimental parser makes them.

The expect on the defining pack stays. between_packs drops constants that no pack defines, and carrying each reference's defining pack beside it would change what between_packs returns to rm too, which doesn't need it. The message now names between_packs.

Test plan

• cargo test (375 passed), cargo fmt --check, and cargo clippy --all-targets --all-features -- -Dwarnings.
• test_list_references_from_associations pins ::Census and ::Taco at carrier.rb:2:2 and :3:2 in app_with_rails_relationships, where the source has only has_many :censuses and has_many :tacos.