Path functions in TSG have unexpected behavior for `.` and `..`

Observed by @nohehf in #437. The behavior of `path-stem` and `path-dir` is unclear on `.` and `..`. One would assume that `(path-join (path-dir path) (path-filename path))` would always return the original path, but that is not the case.

The problem is that `.` and `..` are not considered file names by the underlying Rust path functions, even if they are the only component in the path.