[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.

crates/red_knot_python_semantic/resources/mdtest/import/relative.md

@@ -218,3 +218,33 @@ import package
 # error: [unresolved-attribute] "Type `<module 'package'>` has no attribute `foo`"
 reveal_type(package.foo.X)  # revealed: Unknown
 ```
+
+## In the src-root
+
+`parser.py`:
+
+```py
+X: int = 42
+```
+
+`__main__.py`:
+
+```py
+from .parser import X
+
+reveal_type(X)  # revealed: int
+```
+
+## Beyond the src-root
+
+`parser.py`:
+
+```py
+X: int = 42
+```
+
+`__main__.py`:
+
+```py
+from ..parser import X  # error: [unresolved-import]
+```