fix: adopt forward-compatible approach to `builder:watch`

[danielroe]

🔗 Linked issue

❓ Type of change

• [ ] 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• [x] 🐞 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)
• [ ] 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

We are planning to move to emitting absolute paths in builder:watch in Nuxt v4. You can see a little more about the history in the PR linked in that issue.

This PR is an attempt to use a forward/backward compatible approach, namely resolving the path to ensure it's absolute, then making it relative so your existing code will continue to work.

This should be safe to merge as is, but do let me know if you have any concerns about this approach.

📝 Checklist

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