OS.DirDirectory operations.
exists dir is true if directory dir exists in the file system. Symbolic links are followed.
must_exist dir is dir if file is a regular file in the file system and an error otherwise. Symbolic links are followed.
contents ~dotfiles ~rel dir is the list of directories and filse in dir. If rel is true (defaults to false) the resulting paths are relative to dir, otherwise they have dir prepended. If dotfiles is false (default) elements that start with a . are omitted.