Use lstat() semantics for URL directory detection

[jrflat]

Historically, NS/URL checks if a path is a directory using getattrlist with FSOPT_NOFOLLOW. This is equivalent to lstat() in the sense that it won't follow a symlink at the end of the path, and it will report information about the symlink itself.

In the Swift URL implementation, I called into FileManager to check if a path is a directory, but FileManager uses stat() for this purpose (and historically always has).

Since URL historically doesn't follow a symlink at the end of the path, this PR switches to using lstat() on UNIX-like platforms instead of calling into FileManager.

On Windows, we can create the file handle with FILE_FLAG_OPEN_REPARSE_POINT and check for the flag to achieve the same semantics.

[jrflat]

@swift-ci please test

[jrflat]

@swift-ci please test

[jrflat]

@swift-ci please test

[jrflat]

@swift-ci please test