Motivation:
In #3363 we converted `_NIOFileSystem` to `NIOFileSystem` and removed
the (unreleased) overloads for FilePath/NIOFilePath. This change adds
back `_NIOFileSystem` such that it matches the API it had at 2.86.0.
Modifications:
- Add back `_NIOFileSystem` and `_NIOFileSystemFoundationCompat` such
that their API is at 2.86.0
Result:
- `NIOFileSystem` uses `NIOFilePath`
- `_NIOFileSystem` uses `FilePath`
George Barnett