[red-knot] Add initial support for * imports (#16923)

## Summary

This PR adds initial support for `*` imports to red-knot. The approach
is to implement a standalone query, called from semantic indexing, that
visits the module referenced by the `*` import and collects all
global-scope public names that will be imported by the `*` import. The
`SemanticIndexBuilder` then adds separate definitions for each of these
names, all keyed to the same `ast::Alias` node that represents the `*`
import.

There are many pieces of `*`-import semantics that are still yet to be
done, even with this PR:
- This PR does not attempt to implement any of the semantics to do with
`__all__`. (If a module defines `__all__`, then only the symbols
included in `__all__` are imported, _not_ all public global-scope
symbols.
- With the logic implemented in this PR as it currently stands, we
sometimes incorrectly consider a symbol bound even though it is defined
in a branch that is statically known to be dead code, e.g. (assuming the
target Python version is set to 3.11):

  ```py
  # a.py

  import sys

  if sys.version_info < (3, 10):
      class Foo: ...

  ```

  ```py
  # b.py

  from a import *

  print(Foo)  # this is unbound at runtime on 3.11,
# but we currently consider it bound with the logic in this PR
  ```

Implementing these features is important, but is for now deferred to
followup PRs.

Many thanks to @ntBre, who contributed to this PR in a pairing session
on Friday!

## Test Plan

Assertions in existing mdtests are adjusted, and several new ones are
added.

crates/red_knot_python_semantic/src/semantic_model.rs

@@ -149,7 +149,7 @@ macro_rules! impl_binding_has_ty {
             #[inline]
             fn inferred_type<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
                 let index = semantic_index(model.db, model.file);
-                let binding = index.definition(self);
+                let binding = index.expect_single_definition(self);
                 binding_type(model.db, binding)
             }
         }
@@ -158,10 +158,19 @@ macro_rules! impl_binding_has_ty {
 
 impl_binding_has_ty!(ast::StmtFunctionDef);
 impl_binding_has_ty!(ast::StmtClassDef);
-impl_binding_has_ty!(ast::Alias);
 impl_binding_has_ty!(ast::Parameter);
 impl_binding_has_ty!(ast::ParameterWithDefault);
 
+impl HasType for ast::Alias {
+    fn inferred_type<'db>(&self, model: &SemanticModel<'db>) -> Type<'db> {
+        if &self.name == "*" {
+            return Type::Never;
+        }
+        let index = semantic_index(model.db, model.file);
+        binding_type(model.db, index.expect_single_definition(self))
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use ruff_db::files::system_path_to_file;