## Summary
Related to #15848, this PR adds the imports explicitly as we'll now flag
these symbols as undefined.
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Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
## Summary
Use `Unknown | T_inferred` as the type for *undeclared* public symbols.
## Test Plan
- Updated existing tests
- New test for external `__slots__` modifications.
- New tests for external modifications of public symbols.
