add support for Apps.get_model for migrations

mypy_django_plugin/plugins/migrations.py

@@ -0,0 +1,24 @@
+from typing import cast
+
+from mypy.checker import TypeChecker
+from mypy.nodes import TypeInfo
+from mypy.plugin import MethodContext
+from mypy.types import Type, Instance, TypeType
+
+from mypy_django_plugin import helpers
+
+
+def determine_model_cls_from_string_for_migrations(ctx: MethodContext) -> Type:
+    app_label = ctx.args[ctx.callee_arg_names.index('app_label')][0].value
+    model_name = ctx.args[ctx.callee_arg_names.index('model_name')][0].value
+
+    api = cast(TypeChecker, ctx.api)
+    model_fullname = helpers.get_model_fullname(app_label, model_name, all_modules=api.modules)
+
+    if model_fullname is None:
+        return ctx.default_return_type
+    model_info = helpers.lookup_fully_qualified_generic(model_fullname,
+                                                        all_modules=api.modules)
+    if model_info is None or not isinstance(model_info, TypeInfo):
+        return ctx.default_return_type
+    return TypeType(Instance(model_info, []))