DirExists returns true if a directory exists at `path`, false if `path` does not exist, otherwise an error.

EnsureDir ensures that a directory exists at `path`, and if it must create the directory any necessary parent directories will also be created and the new directory will be empty.

EnsureFile ensures that a regular file exists at `path`, and if it must create the file any necessary parent directories will also be created and the new file will be empty.

FileExists returns true if a regular file exists at `path`, false if `path` does not exist, otherwise an error.

ReplaceFile replaces the contents of the file at `path` with `data` by writing to a tmp file in the same dir as `path` and renaming the tmp file over `path`.