Lint imports and high level design doc

[amaslenn]

Summary

1. Add import linting to make sure _core doesn't depend on any higher-level modules.
2. Rework TestTemplateStrategy to avoid dependency on SlurmSystem.
3. Add initial design description.

Test Plan

CI

Additional Notes

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[TaekyungHeo]

Add import linting to make sure _core doesn't depend on any higher-level modules.

I have not used lint-imports before, but it looks like a good idea.