test: improve external modules test

[pi0]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

While working on #527, tests was not enough to test edge cases. This PR improves node_modules testing fixture:

• Use top level node_modules
• Use @fixture scoped packages
• Introduce new fixture with subpath export (useful to followup on feat: resolve actual subpaths before guessing #527 and alike)
• Add basic chunk checks for node

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.