fix: adopt forward-compatible approach to builder:watch

[danielroe]

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.31%. Comparing base (b41470d) to head (887d014).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #476      +/-   ##
==========================================
+ Coverage   42.22%   42.31%   +0.08%     
==========================================
  Files          26       26              
  Lines        1383     1385       +2     
  Branches      139      139              
==========================================
+ Hits          584      586       +2     
- Misses        778      779       +1     
+ Partials       21       20       -1     

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