Match and report an unused_pub item by its position, not by its key

[robobun]

Found through oven-sh/bun, where the mordant job runs over three targets and its baseline check passed or failed at random.

Problem

• A run over several targets records one item once per target, and not always under one key. The definition path numbers the impl blocks of a module, so a cfg(windows) block above an item gives it {impl#10} on Windows and {impl#9} on Linux and macOS.
• A use recorded under the Windows key does not count for the Linux record of the same item. An item that only Windows code calls is reported as unused.
• judge removes duplicates with dedup_by on the key, which removes only neighbours. With the records of three targets in the order A, B, A nothing is removed, with A, A, B one is. The order is the order in which cargo finishes the targets, so the number of findings changes from run to run over the same code. bun counted 65 or 68 findings in one file, against a baseline of 65.

Fix

• judge takes an item as used when a use of any of its keys was recorded. It groups the records by Def::position_key() (the crate plus the file and offset of the name), which is the same in every unit.
• It keeps one finding per position: the one with the smallest key, so the choice does not depend on the order of the units.
• The "item of an unused type goes with it" step still uses keys, because parent is the key that the item's own unit gave the type.
• Test: unused_pub_knows_one_item_under_the_keys_of_two_targets, a library with a cfg(windows) impl block above another impl block, and a binary that calls into it under cfg(windows). Without the change it reports by_windows (a false positive) and reports by_nothing twice.

Checked

• cargo fmt --check, cargo test, cargo clippy --all-targets -- -D warnings, and mordant over itself with -D warnings.
• Over oven-sh/bun, three targets, no baseline: unused_pub reports go from 188 to 173. 18 items are no longer reported, and I checked a sample: each has a real use under one target (Tag::epoll_ctl on Linux, Tag::uv_spawn on Windows, FilePollRef::set_flag, ExtraPipe::fd). The three items that were reported two or three times are reported once. Nothing new is reported. The pairs that still share a message are separate items: bun_sys::O::NOATIME is defined at three places under different cfgs.