Fix wrong unused-import when referenced in __all__

[maldoinc]

Summary

Currently if a file contains an import that is only referenced in __all__ it is incorrectly flagged as unused. This addresses that.

Fixes #2128

Test Plan

Added test_diagnostic_import_used_in_all.

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[stroxler]

This looks reasonable to me, thanks! It might be good to file an issue for this as well, just to track the fix.

More importantly, we can't merge without the CLA signed - once it is, I can get a second review and try to merge

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

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[maldoinc]

@stroxler Created #2128.