ignore_missing_imports removed from mypy.ini not noticed due to cache.

• Are you reporting a bug, or opening a feature request?

bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

$ cat >test.py <<__EOF__
import foobarxyz
__EOF__

$ cat >mypy.ini <<__EOF__
[mypy]
[mypy-foobarxyz]
ignore_missing_imports = True
__EOF__

$ mypy test.py
--> passes typecheck - GOOD

$ cat >mypy.ini <<__EOF__
[mypy]
__EOF__

$ mypy test.py
--> passes typecheck - BAD (*)

$ rm -R .mypy_cache
$ mypy test.py
--> fails typecheck - GOOD

• What is the actual behavior/output?
  In (*), the file passes typecheck even if we removed ignore_missing_imports from config, which should result in an unresolved import.

• What is the behavior/output you expect?
  A failed typecheck - the result with cache should be the same as without.

• What are the versions of mypy and Python you are using?
  0.770

issue の test.py と mypy.ini のコマンドで動作を再現し、その後、キャッシュを使用した実行と .mypy_cache を削除した後の実行を比較します。設定の変更がキャッシュにどのように影響するかを追跡し、ignore_missing_imports を削除すると、既存のキャッシュがある場合とない場合で同じ未解決インポートの結果になることを確認します。