[red-knot] Fix relative imports in src.root (#15990)

## Summary Fixes #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.

Author: MichaReiser

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]
+```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -2513,18 +2513,32 @@ impl<'db> TypeInferenceBuilder<'db> {
  .ok_or(ModuleNameResolutionError::UnknownCurrentModule)?;
  let mut level = level.get();
  if module.kind().is_package() {
- level -= 1;
+ level = level.saturating_sub(1);
  }
+
  let mut module_name = module.name().clone();
- for _ in 0..level {
- module_name = module_name
- .parent()
- .ok_or(ModuleNameResolutionError::TooManyDots)?;
+ let tail = tail
+ .map(|tail| ModuleName::new(tail).ok_or(ModuleNameResolutionError::InvalidSyntax))
+ .transpose()?;
+
+ for remaining_dots in (0..level).rev() {
+ if let Some(parent) = module_name.parent() {
+ module_name = parent;
+ } else if remaining_dots == 0 {
+ // If we reached a search path root and this was the last dot return the tail if any.
+ // If there's no tail, then we have a relative import that's too deep.
+ return tail.ok_or(ModuleNameResolutionError::TooManyDots);
+ } else {
+ // We're at a search path root. This is a too deep relative import if there's more than
+ // one dot remaining.
+ return Err(ModuleNameResolutionError::TooManyDots);
+ }
  }
+
  if let Some(tail) = tail {
- let tail = ModuleName::new(tail).ok_or(ModuleNameResolutionError::InvalidSyntax)?;
  module_name.extend(&tail);
  }
+
  Ok(module_name)
  }