chore: additional test coverage for substacks

[eladb]

add a few more tests to cover behaviors of substacks (stacks that are
defined within the scope of another stack) to cover:

• Same-environment cross stack references
• Stack.of() finds the right stack
• PhysicalName.GENERATE_IF_NEEDED is triggered

---

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[skinny85]

Fixed the out of order imports that were making the build fail.

[mergify]

Thanks so much for taking the time to contribute to the AWS CDK ❤️

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checklist:

• PR title type(scope): text
  • type: fix, feat, refactor go into CHANGELOG, chore is hidden
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• PR Description
  • Rationale: describe rationale of change and approach taken
  • Issues: indicate issues fixed via: fixes #xxx or closes #xxx
  • Breaking?: last paragraph: BREAKING CHANGE: <describe what changed + link for details>
• Testing
  • Unit test added. Prefer to add a new test rather than modify existing tests
  • CLI or init templates change? Re-run/add CLI integration tests
• Documentation
  • README: update module README to describe new features
  • API docs: public APIs must be documented. Copy from official AWS docs when possible
  • Design: for significant features, follow design process