Skip symbolic links on POSIX platforms

I have a fairly deep source tree and I leave symlinks around pointing to the top of the tree for convenience. Unfortunately this means dirToFiles never terminates, and I don't see a workaround.

System.Directory.doesDirectoryExist cannot discriminate between symlinks and real directories, so on non-Windows platforms we use System.Posix.Files.getSymbolicLinkStatus instead.

Bonus commits to fix upstream repository URL and a minor refactor of dirToFiles.