[flake8-pyi] Add autofix for PYI058 (#9355)

## Summary

This PR adds an autofix for the newly added PYI058 rule (added in
#9313). ~~The PR's current implementation is that the fix is only
available if the fully qualified name of `Generator` or `AsyncGenerator`
is being used:~~
- ~~`-> typing.Generator` is converted to `-> typing.Iterator`;~~
- ~~`-> collections.abc.AsyncGenerator[str, Any]` is converted to `->
collections.abc.AsyncIterator[str]`;~~
- ~~but `-> Generator` is _not_ converted to `-> Iterator`. (It would
require more work to figure out if `Iterator` was already imported or
not. And if it wasn't, where should we import it from? `typing`,
`typing_extensions`, or `collections.abc`? It seems much more
complicated.)~~

The fix is marked as always safe for `__iter__` or `__aiter__` methods
in `.pyi` files, but unsafe for all such methods in `.py` files that
have more than one statement in the method body.

This felt slightly fiddly to accomplish, but I couldn't _see_ any
utilities in
https://github.com/astral-sh/ruff/tree/main/crates/ruff_linter/src/fix
that would have made it simpler to implement. Lmk if I'm missing
something, though -- my first time implementing an autofix! :)

## Test Plan

`cargo test` / `cargo insta review`.

crates/ruff_linter/resources/test/fixtures/flake8_pyi/PYI058.py

@@ -1,82 +1,174 @@
-import collections.abc
-import typing
-from collections.abc import AsyncGenerator, Generator
-from typing import Any
+def scope():
+    from collections.abc import Generator
 
-class IteratorReturningSimpleGenerator1:
-    def __iter__(self) -> Generator:  # PYI058 (use `Iterator`)
-        return (x for x in range(42))
+    class IteratorReturningSimpleGenerator1:
+        def __iter__(self) -> Generator:
+            ...  # PYI058 (use `Iterator`)
 
-class IteratorReturningSimpleGenerator2:
-    def __iter__(self, /) -> collections.abc.Generator[str, Any, None]:  # PYI058 (use `Iterator`)
-        """Fully documented, because I'm a runtime function!"""
-        yield from "abcdefg"
-        return None
 
-class IteratorReturningSimpleGenerator3:
-    def __iter__(self, /) -> collections.abc.Generator[str, None, typing.Any]:  # PYI058 (use `Iterator`)
-        yield "a"
-        yield "b"
-        yield "c"
-        return
+def scope():
+    import typing
 
-class AsyncIteratorReturningSimpleAsyncGenerator1:
-    def __aiter__(self) -> typing.AsyncGenerator: pass # PYI058 (Use `AsyncIterator`)
+    class IteratorReturningSimpleGenerator2:
+        def __iter__(self) -> typing.Generator:
+            ...  # PYI058 (use `Iterator`)
 
-class AsyncIteratorReturningSimpleAsyncGenerator2:
-    def __aiter__(self, /) -> collections.abc.AsyncGenerator[str, Any]: ...  # PYI058 (Use `AsyncIterator`)
 
-class AsyncIteratorReturningSimpleAsyncGenerator3:
-    def __aiter__(self, /) -> collections.abc.AsyncGenerator[str, None]: pass  # PYI058 (Use `AsyncIterator`)
+def scope():
+    import collections.abc
 
-class CorrectIterator:
-    def __iter__(self) -> Iterator[str]: ...  # OK
+    class IteratorReturningSimpleGenerator3:
+        def __iter__(self) -> collections.abc.Generator:
+            ...  # PYI058 (use `Iterator`)
 
-class CorrectAsyncIterator:
-    def __aiter__(self) -> collections.abc.AsyncIterator[int]: ...  # OK
 
-class Fine:
-    def __iter__(self) -> typing.Self: ...  # OK
+def scope():
+    import collections.abc
+    from typing import Any
 
-class StrangeButWeWontComplainHere:
-    def __aiter__(self) -> list[bytes]: ...  # OK
+    class IteratorReturningSimpleGenerator4:
+        def __iter__(self, /) -> collections.abc.Generator[str, Any, None]:
+            ...  # PYI058 (use `Iterator`)
 
-def __iter__(self) -> Generator: ...  # OK (not in class scope)
-def __aiter__(self) -> AsyncGenerator: ...  # OK (not in class scope)
 
-class IteratorReturningComplexGenerator:
-    def __iter__(self) -> Generator[str, int, bytes]: ...  # OK
+def scope():
+    import collections.abc
+    import typing
 
-class AsyncIteratorReturningComplexAsyncGenerator:
-    def __aiter__(self) -> AsyncGenerator[str, int]: ...  # OK
+    class IteratorReturningSimpleGenerator5:
+        def __iter__(self, /) -> collections.abc.Generator[str, None, typing.Any]:
+            ...  # PYI058 (use `Iterator`)
 
-class ClassWithInvalidAsyncAiterMethod:
-    async def __aiter__(self) -> AsyncGenerator: ...  # OK
 
-class IteratorWithUnusualParameters1:
-    def __iter__(self, foo) -> Generator: ...  # OK
+def scope():
+    from collections.abc import Generator
 
-class IteratorWithUnusualParameters2:
-    def __iter__(self, *, bar) -> Generator: ...  # OK
+    class IteratorReturningSimpleGenerator6:
+        def __iter__(self, /) -> Generator[str, None, None]:
+            ...  # PYI058 (use `Iterator`)
 
-class IteratorWithUnusualParameters3:
-    def __iter__(self, *args) -> Generator: ...  # OK
 
-class IteratorWithUnusualParameters4:
-    def __iter__(self, **kwargs) -> Generator: ...  # OK
+def scope():
+    import typing_extensions
 
-class IteratorWithIterMethodThatReturnsThings:
-    def __iter__(self) -> Generator:  # OK
-        yield
-        return 42
+    class AsyncIteratorReturningSimpleAsyncGenerator1:
+        def __aiter__(
+            self,
+        ) -> typing_extensions.AsyncGenerator:
+            ...  # PYI058 (Use `AsyncIterator`)
 
-class IteratorWithIterMethodThatReceivesThingsFromSend:
-    def __iter__(self) -> Generator:  # OK
-        x = yield 42
 
-class IteratorWithNonTrivialIterBody:
-    def __iter__(self) -> Generator:  # OK
-        foo, bar, baz = (1, 2, 3)
-        yield foo
-        yield bar
-        yield baz
+def scope():
+    import collections.abc
+
+    class AsyncIteratorReturningSimpleAsyncGenerator2:
+        def __aiter__(self, /) -> collections.abc.AsyncGenerator[str, Any]:
+            ...  # PYI058 (Use `AsyncIterator`)
+
+
+def scope():
+    import collections.abc
+
+    class AsyncIteratorReturningSimpleAsyncGenerator3:
+        def __aiter__(self, /) -> collections.abc.AsyncGenerator[str, None]:
+            ...  # PYI058 (Use `AsyncIterator`)
+
+
+def scope():
+    from typing import Iterator
+
+    class CorrectIterator:
+        def __iter__(self) -> Iterator[str]:
+            ...  # OK
+
+
+def scope():
+    import collections.abc
+
+    class CorrectAsyncIterator:
+        def __aiter__(self) -> collections.abc.AsyncIterator[int]:
+            ...  # OK
+
+
+def scope():
+    import typing
+
+    class Fine:
+        def __iter__(self) -> typing.Self:
+            ...  # OK
+
+
+def scope():
+    class StrangeButWeWontComplainHere:
+        def __aiter__(self) -> list[bytes]:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import Generator
+
+    def __iter__(self) -> Generator:
+        ...  # OK (not in class scope)
+
+
+def scope():
+    from collections.abc import AsyncGenerator
+
+    def __aiter__(self) -> AsyncGenerator:
+        ...  # OK (not in class scope)
+
+
+def scope():
+    from collections.abc import Generator
+
+    class IteratorReturningComplexGenerator:
+        def __iter__(self) -> Generator[str, int, bytes]:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import AsyncGenerator
+
+    class AsyncIteratorReturningComplexAsyncGenerator:
+        def __aiter__(self) -> AsyncGenerator[str, int]:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import AsyncGenerator
+
+    class ClassWithInvalidAsyncAiterMethod:
+        async def __aiter__(self) -> AsyncGenerator:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import Generator
+
+    class IteratorWithUnusualParameters1:
+        def __iter__(self, foo) -> Generator:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import Generator
+
+    class IteratorWithUnusualParameters2:
+        def __iter__(self, *, bar) -> Generator:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import Generator
+
+    class IteratorWithUnusualParameters3:
+        def __iter__(self, *args) -> Generator:
+            ...  # OK
+
+
+def scope():
+    from collections.abc import Generator
+
+    class IteratorWithUnusualParameters4:
+        def __iter__(self, **kwargs) -> Generator:
+            ...  # OK