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

Riproduci il comportamento con i comandi test.py e mypy.ini dell'issue, quindi confronta l'esecuzione con la cache con l'esecuzione dopo aver rimosso .mypy_cache. Traccia il modo in cui le modifiche alla configurazione influiscono sulla cache e verifica che la rimozione di ignore_missing_imports produca lo stesso risultato di importazione non risolta con e senza una cache esistente.