distinguish references from definitions in infer_nonlocal

The initial implementation of `infer_nonlocal` landed in
https://github.com/astral-sh/ruff/pull/19112 fails to report an error
for this example:

```py
x = 1
def f():
    # This is only a usage of `x`, not a definition. It shouldn't be
    # enough to make the `nonlocal` statement below allowed.
    print(x)
    def g():
        nonlocal x
```

Fix this by continuing to walk enclosing scopes when the place we've
found isn't bound, declared, or `nonlocal`.

crates/ty_python_semantic/src/semantic_index/place.rs

@@ -320,7 +320,7 @@ impl PlaceExprWithFlags {
         self.flags.contains(PlaceFlags::IS_USED)
     }
 
-    /// Is the place defined in its containing scope?
+    /// Is the place given a value in its containing scope?
     pub fn is_bound(&self) -> bool {
         self.flags.contains(PlaceFlags::IS_BOUND)
     }