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

[hendrikvanantwerpen]

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.