unexpected attr-defined error in imports in dead code

Bug Report

mypy still gives an attr-defined error in dead code when importing platform-specific attributes from modules.

To Reproduce

import sys

if sys.platform != "win32":
    assert False

from ctypes import windll

Check it on any OS other than Windows.

Expected Behavior

No errors. This is valid code since it has the platform-specific guard.

Actual Behavior

$ uvx mypy example.py 
example.py:6: error: Module "ctypes" has no attribute "windll"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 1.19.0 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12

Reproduce the report using the example.py snippet and uvx mypy example.py on a non-Windows system. Trace mypy's handling of platform-specific imports in dead code, then add focused coverage for this case if the relevant test location is found. Done means the guarded from ctypes import windll no longer produces an attr-defined error.