`isDirectory(at:)` discards prefetched resource values

`contentsOfDirectory(at:includingPropertiesForKeys:)` is called with `[.contentTypeKey, .isDirectoryKey]` — precisely so those values are cached on the returned URLs. But `isDirectory(at:)` calls `fileExists(atPath:isDirectory:)`, a fresh syscall that ignores the cache, once per URL in the import loop.

Reading `url.resourceValues(forKeys: [.isDirectoryKey]).isDirectory` would use the prefetched value. (`url.contentType` already does the right thing.)

Start at the import loop that calls isDirectory(at:) after contentsOfDirectory(at:includingPropertiesForKeys:) prefetched .isDirectoryKey and .contentTypeKey. Compare it with url.contentType and read the resource value from each URL instead of making a fresh fileExists(atPath:isDirectory:) syscall. Done means the cached directory value is used for each URL while the import behavior remains unchanged.

Written by the indexing model from the issue text.