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

Reproduce the behavior with the issue's test.py and mypy.ini commands, then compare the cached run with the run after removing .mypy_cache. Trace how configuration changes affect the cache, and verify that removing ignore_missing_imports produces the same unresolved-import result with and without an existing cache.