[red-knot] Fix relative imports in `src.root` (#15990)
## Summary
Fixes https://github.com/astral-sh/ruff/issues/15989
Red Knot failed to resolve relative imports if the importing module is
located at a search path root.
The issue was that the module resolver returned an `Err(TooManyDots)` as
soon as the parent of the current module is `None` (which is the case
for a module at the search path root).
However, this is incorrect if a `tail` (a module name) exists.
