Fix #224 add paths in mypy's mypy_path option or MYPYPATH env var for import discovery (#498)

test-data/typecheck/test_config.yml

@@ -38,3 +38,30 @@
               class MyModel(models.Model):
                   user = models.ForeignKey('auth.User', on_delete=models.CASCADE)
 
+-   case: add_mypy_path_to_package_search
+    main: |
+        import extra_module
+    mypy_config: |
+        [mypy]
+        mypy_path = ./extras
+        [mypy.plugins.django-stubs]
+        django_settings_module = mysettings
+    files:
+        -   path: extras/extra_module.py
+            content: |
+                def extra_fn():
+                  pass
+
+-   case: add_mypypath_env_var_to_package_search
+    main: |
+        import extra_module
+    mypy_config: |
+        [mypy.plugins.django-stubs]
+        django_settings_module = mysettings
+    env:
+      - MYPYPATH=./extras
+    files:
+        -   path: extras/extra_module.py
+            content: |
+                def extra_fn():
+                  pass