Support type[a.X] with qualified class names (#14825)

This adds support for `type[a.X]`, where the `type` special form is
applied to a qualified name that resolves to a class literal. This works
for both nested classes and classes imported from another module.

Closes #14545

crates/red_knot_python_semantic/resources/mdtest/type_of/basic.md

@@ -0,0 +1,89 @@
+# type special form
+
+## Class literal
+
+```py
+class A: ...
+
+def f() -> type[A]:
+    return A
+
+reveal_type(f())  # revealed: type[A]
+```
+
+## Nested class literal
+
+```py
+class A:
+    class B: ...
+
+def f() -> type[A.B]:
+    return A.B
+
+reveal_type(f())  # revealed: type[B]
+```
+
+## Deeply nested class literal
+
+```py
+class A:
+    class B:
+        class C: ...
+
+def f() -> type[A.B.C]:
+    return A.B.C
+
+reveal_type(f())  # revealed: type[C]
+```
+
+## Class literal from another module
+
+```py
+from a import A
+
+def f() -> type[A]:
+    return A
+
+reveal_type(f())  # revealed: type[A]
+```
+
+```py path=a.py
+class A: ...
+```
+
+## Qualified class literal from another module
+
+```py
+import a
+
+def f() -> type[a.B]:
+    return a.B
+
+reveal_type(f())  # revealed: type[B]
+```
+
+```py path=a.py
+class B: ...
+```
+
+## Deeply qualified class literal from another module
+
+```py path=a/test.py
+import a.b
+
+# TODO: no diagnostic
+# error: [unresolved-attribute]
+def f() -> type[a.b.C]:
+    # TODO: no diagnostic
+    # error: [unresolved-attribute]
+    return a.b.C
+
+reveal_type(f())  # revealed: @Todo(unsupported type[X] special form)
+```
+
+```py path=a/__init__.py
+```
+
+```py path=a/b.py
+class C: ...
+```

crates/red_knot_python_semantic/src/types/infer.rs

@@ -4655,18 +4655,18 @@ impl<'db> TypeInferenceBuilder<'db> {
     /// Given the slice of a `type[]` annotation, return the type that the annotation represents
     fn infer_subclass_of_type_expression(&mut self, slice: &ast::Expr) -> Type<'db> {
         match slice {
-            ast::Expr::Name(_) => {
+            ast::Expr::Name(_) | ast::Expr::Attribute(_) => {
                 let name_ty = self.infer_expression(slice);
                 if let Some(ClassLiteralType { class }) = name_ty.into_class_literal() {
                     Type::subclass_of(class)
                 } else {
-                    todo_type!()
+                    todo_type!("unsupported type[X] special form")
                 }
             }
-            // TODO: attributes, unions, subscripts, etc.
+            // TODO: unions, subscripts, etc.
             _ => {
                 self.infer_type_expression(slice);
-                todo_type!()
+                todo_type!("unsupported type[X] special form")
             }
         }
     }