Do not display import-related errors after module-level always false assert

[sterliakov]

Fixes #19346

[A5rocks]

One reason I'm surprised about this is that my impression was that after a module-level assert that is unreachable, mypy removes all nodes. Am I misremembering? Or is this an earlier pass?

[sterliakov]

@A5rocks Great question. Imports are collected during parsing, and "definitely unreachable" truncation step follows (SemanticAnalyzerPreAnalysis). It does not modify MypyFile.imports collected at the previous step, so we still treat such modules as dependencies even if the corresponding import statement no longer exists, and "missing module" errors are produced by build manager when any dependency can't be resolved. Those imports aren't marked unreachable either. That probably should be fixed instead, thanks! That will be a better approach.

[A5rocks]

This makes sense to me!

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

PyWinCtl (https://github.com/Kalmat/PyWinCtl)
- src/pywinctl/_pywinctl_win.py:28: error: Cannot find implementation or library stub for module named "pywinbox"  [import-not-found]
- src/pywinctl/_pywinctl_macos.py:25: error: Cannot find implementation or library stub for module named "pywinbox"  [import-not-found]