[pyflakes] Visit forward annotations in TypeAliasType as types (F401) (#15829)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/15812 by visiting the
second argument as a type definition.

## Test Plan

New F401 tests based on the report.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

crates/ruff_python_semantic/src/analyze/typing.rs

@@ -28,6 +28,7 @@ pub enum Callable {
     NamedTuple,
     TypedDict,
     MypyExtension,
+    TypeAliasType,
 }
 
 #[derive(Debug, Copy, Clone)]