django-stubs-ext: Export RelatedManager, ManyRelatedManager stub-only classes (#1834)

RelatedManager, ManyRelatedManager classes are sometimes useful for type hinting.
But these classes don't exist as is at Django runtime, rather they are defined inside function bodies.

* When `TYPE_CHECKING`, we re-export django-stubs fake classes.
* At runtime, we define these as `Protocol[_T]`.
  This has the advantage that Python prevents them being used with isinstance().

Usage before:

```python
if TYPE_CHECKING:
    from django.db.models.manager import RelatedManager  # before PR #1814

def get_manager() -> "RelatedManager[MyModel]": ...
```

Usage after:

```python
from django_stubs_ext.db.models.manager import RelatedManager

def get_manager() -> RelatedManager[MyModel]: ...
```

Author: intgr

ext/django_stubs_ext/db/models.py renamed to ext/django_stubs_ext/db/models/__init__.py

@@ -0,0 +1,23 @@
+from typing import TYPE_CHECKING
+
+# Re-export stubs-only classes RelatedManger and ManyRelatedManager.
+# These are fake, Django defines these inside function body.
+if TYPE_CHECKING:
+    # noinspection PyUnresolvedReferences
+    from django.db.models.fields.related_descriptors import ManyRelatedManager as ManyRelatedManager
+
+    # noinspection PyUnresolvedReferences
+    from django.db.models.fields.related_descriptors import RelatedManager as RelatedManager
+
+else:
+    from typing import Protocol, TypeVar
+
+    _T = TypeVar("_T")
+
+    # Define as `Protocol` to prevent them being used with `isinstance()`.
+    # These actually inherit from `BaseManager`.
+    class RelatedManager(Protocol[_T]):
+        pass
+
+    class ManyRelatedManager(Protocol[_T]):
+        pass