Treat typing.Annotated subscripts as type definitions (#10285)

## Summary

I think this code has existed since the start of `typing.Annotated`
support (#333), and was then
overlooked over a series of refactors.

Closes #10279.

Author: charliermarsh

crates/ruff_linter/resources/test/fixtures/pyflakes/F401_23.py

@@ -0,0 +1,7 @@
+"""Test: ensure that we treat strings in `typing.Annotation` as type definitions."""
+
+from pathlib import Path
+from re import RegexFlag
+from typing import Annotated
+
+p: Annotated["Path", int] = 1

crates/ruff_linter/src/checkers/ast/mod.rs

@@ -1348,7 +1348,7 @@ impl<'a> Visitor<'a> for Checker<'a> {
                             {
                                 let mut iter = elts.iter();
                                 if let Some(expr) = iter.next() {
-                                    self.visit_expr(expr);
+                                    self.visit_type_definition(expr);
                                 }
                                 for expr in iter {
                                     self.visit_non_type_definition(expr);

crates/ruff_linter/src/rules/pyflakes/mod.rs

@@ -55,6 +55,7 @@ mod tests {
     #[test_case(Rule::UnusedImport, Path::new("F401_20.py"))]
     #[test_case(Rule::UnusedImport, Path::new("F401_21.py"))]
     #[test_case(Rule::UnusedImport, Path::new("F401_22.py"))]
+    #[test_case(Rule::UnusedImport, Path::new("F401_23.py"))]
     #[test_case(Rule::ImportShadowedByLoopVar, Path::new("F402.py"))]
     #[test_case(Rule::ImportShadowedByLoopVar, Path::new("F402.ipynb"))]
     #[test_case(Rule::UndefinedLocalWithImportStar, Path::new("F403.py"))]

crates/ruff_linter/src/rules/pyflakes/snapshots/ruff_linter__rules__pyflakes__tests__F401_F401_23.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff_linter/src/rules/pyflakes/mod.rs
+---
+F401_23.py:4:16: F401 [*] `re.RegexFlag` imported but unused
+  |
+3 | from pathlib import Path
+4 | from re import RegexFlag
+  |                ^^^^^^^^^ F401
+5 | from typing import Annotated
+  |
+  = help: Remove unused import: `re.RegexFlag`
+
+ℹ Safe fix
+1 1 | """Test: ensure that we treat strings in `typing.Annotation` as type definitions."""
+2 2 | 
+3 3 | from pathlib import Path
+4   |-from re import RegexFlag
+5 4 | from typing import Annotated
+6 5 | 
+7 6 | p: Annotated["Path", int] = 1