Fix/cleanup "path", "file", "dir" types, step 2
Fix/cleanup "path", "file", "dir" types, step 2

Use `os.path.relpath` to compute relative paths, normalize paths
everywhere, and check only at one location if relative path goes "up"
too much. It's much more robust and clear.

Also cleanup error messages: both original value and computed value.

Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake,Deepomatic/dmake