Incremental mode confusion about deleted module imported with import from

[msullivan]

The following test produces no errors:

[case testDeletedModuleFromImport]
# flags: --ignore-missing-imports
from a import b
[file a/__init__.py]
[file a/b.py]
[delete a/b.py.2]

while it would if not incremental, saying that a has no attribute b.

The issue here is that the cache indicates that a.b is a module, and so we think of it as a missing module import, which we ignore, and not a missing attribute import, which we report.

This caused a divergence between incremental and full modes that broke our build.

(This seems similar to #4698 but I don't /think/ it is actually related.)