Reorganize code a bit, add current directory to sys.path (#198)

* reorganize code a bit

* add current directory to sys.path

* remove PYTHONPATH mention from the docs

* linting

mypy_django_plugin/transformers/orm_lookups.py

@@ -0,0 +1,51 @@
+from mypy.plugin import MethodContext
+from mypy.types import AnyType, Instance
+from mypy.types import Type as MypyType
+from mypy.types import TypeOfAny
+
+from mypy_django_plugin.django.context import DjangoContext
+from mypy_django_plugin.lib import fullnames, helpers
+
+
+def typecheck_queryset_filter(ctx: MethodContext, django_context: DjangoContext) -> MypyType:
+    lookup_kwargs = ctx.arg_names[1]
+    provided_lookup_types = ctx.arg_types[1]
+
+    assert isinstance(ctx.type, Instance)
+
+    if not ctx.type.args or not isinstance(ctx.type.args[0], Instance):
+        return ctx.default_return_type
+
+    model_cls_fullname = ctx.type.args[0].type.fullname()
+    model_cls = django_context.get_model_class_by_fullname(model_cls_fullname)
+    if model_cls is None:
+        return ctx.default_return_type
+
+    for lookup_kwarg, provided_type in zip(lookup_kwargs, provided_lookup_types):
+        if lookup_kwarg is None:
+            continue
+        if (isinstance(provided_type, Instance)
+                and provided_type.type.has_base('django.db.models.expressions.Combinable')):
+            provided_type = resolve_combinable_type(provided_type, django_context)
+
+        lookup_type = django_context.resolve_lookup_expected_type(ctx, model_cls, lookup_kwarg)
+        # Managers as provided_type is not supported yet
+        if (isinstance(provided_type, Instance)
+                and helpers.has_any_of_bases(provided_type.type, (fullnames.MANAGER_CLASS_FULLNAME,
+                                                                  fullnames.QUERYSET_CLASS_FULLNAME))):
+            return ctx.default_return_type
+
+        helpers.check_types_compatible(ctx,
+                                       expected_type=lookup_type,
+                                       actual_type=provided_type,
+                                       error_message=f'Incompatible type for lookup {lookup_kwarg!r}:')
+
+    return ctx.default_return_type
+
+
+def resolve_combinable_type(combinable_type: Instance, django_context: DjangoContext) -> MypyType:
+    if combinable_type.type.fullname() != fullnames.F_EXPRESSION_FULLNAME:
+        # Combinables aside from F expressions are unsupported
+        return AnyType(TypeOfAny.explicit)
+
+    return django_context.resolve_f_expression_type(combinable_type)