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`.
