Add test to import all modules to check validity of stubs (#56)

* add import_all.test builder

* fix errors

* fix typechecking errors

* fix migrations typechecking

scripts/build_import_all_test.py

@@ -0,0 +1,29 @@
+import os
+from pathlib import Path
+from typing import List
+
+
+STUBS_ROOT = Path(__file__).parent.parent / 'django-stubs'
+
+
+def build_package_name(path: str) -> str:
+    return '.'.join(['django'] + list(Path(path).relative_to(STUBS_ROOT).with_suffix('').parts))
+
+
+packages: List[str] = []
+for dirpath, dirnames, filenames in os.walk(STUBS_ROOT):
+    if not dirnames:
+        package = build_package_name(dirpath)
+        packages.append(package)
+
+    for filename in filenames:
+        if filename != '__init__.pyi':
+            package = build_package_name(os.path.join(dirpath, filename))
+            packages.append(package)
+
+test_lines: List[str] = []
+for package in packages:
+    test_lines.append('import ' + package)
+
+test_contents = '\n'.join(test_lines)
+print(test_contents)