Worktree 0.17.0

• archive, finish --archive, discard-cache (and finish --discard-cache, sweep) and
  prune-archives --strip-build-output recognise a Flutter app's build output by one I/O-free
  classifier in the domain (flutter_output_kind, flutter_output), from the observations each
  caller supplies (FlutterFacts): whether <P>/pubspec.yaml is present beside the directory as
  anything but a directory, and whether anything is tracked at its path. The strip's behaviour is
  unchanged.
• archive leaves out each file neither HEAD nor the index tracks strictly below <P>/build/ or
  <P>/.dart_tool/, when <P>/pubspec.yaml is a regular file on disk (tracked or not), the
  directory is a real directory, and neither HEAD's tree nor the index holds anything at or below
  it. The directory is recorded as a build_output target with origin archive; the outermost of
  a cargo target and a Flutter output directory is recorded. No manifest field is added and
  format 3 is unchanged on the wire. Removal through such an archive re-observes each left-out file
  immediately before deleting it, as a file or symlink below the same recorded directory, still
  recognised; anything else refuses.
• Verification of a format 3 archive leaves build layout out only below the directories its
  manifest records, so an archive 0.16.0 wrote of a tree holding a Flutter app's output (which it
  kept in dirty.patch and the fingerprint) still verifies, and removal through it still works.
  Layout below a directory the manifest does not record stays in the fingerprint and refuses as
  archive-stale, naming the directory.
• discard-cache deletes an ignored, real <P>/build/ or <P>/.dart_tool/ the classifier
  recognises, as the new cache kind flutter-output (CacheKind::FlutterOutput); a build/
  with no pubspec.yaml beside it, and every other ignored entry, is retained. Cargo behaviour is
  unchanged.
• discard-cache retains a Flutter output directory whole when a .git (a file, a directory or a
  symbolic link) lies at any depth below it, found by a walk that follows no symbolic link and
  repeated immediately before deletion; a directory the walk cannot read is retained too. A
  repository there may hold commits no remote does; archive images it.
• A directory that is both a recognised cargo target and a Flutter output directory, such as a
  cargo target named build/ beside a pubspec.yaml, is classified by the cargo rule alone in
  archive, discard-cache and the strip: the classifier takes whether the directory is a cargo
  target as one of its observations (FlutterFacts::cargo_target), so a file below it that is not
  cargo layout stays archived, as discard-cache keeps it.
• archive prints left out <files> file(s), <bytes> bytes of build output under <directory>,
  and the strip's summary bytes of build output strippable; both said "cargo build output".