mypy should not follow imports from stubs with --follow-imports=skip

[ilinum]

Currently, even with --follow-imports=skip imports from stub files are still followed. This means that if stub imports an unanalyzed module, the previously unanalyzed module is actually typechecked.

Consider the following example:

# main.py
from stub import x

# stub.pyi
from unanalyzed import x

# unanalyzed.py
x: int = 42

x + ''  # error is reported here but it shouldn't be because this file is unanalyzed

Currently, if you run mypy the following happens:

$ mypy main.py --follow-imports=skip
unanalyzed.py:4: error: Unsupported operand types for + ("int" and "str")

I think we should treat imports from stub and non-stub files the same way. Following imports from stubs in silent mode was implemented #1372.

Discovered this bug when investigating an error in Dropbox internal codebases.

[gvanrossum]

Hm, reading through the original discussion in #1364 and #1372 I agree that this is an iffy feature and I'm fine with rolling it back. I don't think this was ever documented -- I don't even recall being aware of this corner case when I refactored --silent/-s into --follow-imports=<enum> and --ignore-missing-imports.