lib: resolve sources of index source maps

[lazerg]

Fixes: #66657

sourcesToAbsolute() reads the top-level sources of every map it loads. Index maps only have sections, so it throws, the map is never cached, and both --enable-source-maps and findSourceMap() ignore it.

This resolves the sources of each section's map instead, using that map's sourceRoot. Test runner coverage reads the same top-level field and would hit the same error once these maps load, so it now walks the sections too.

[nodejs-github-bot]

Review requested:

• @nodejs/test_runner

[github-actions]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.46%. Comparing base (cbac135) to head (f6a315b).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/test_runner/coverage.js	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #66659      +/-   ##
==========================================
- Coverage   90.47%   90.46%   -0.01%     
==========================================
  Files         791      791              
  Lines      277088   277092       +4     
  Branches    53276    53280       +4     
==========================================
- Hits       250686   250682       -4     
+ Misses      16813    16807       -6     
- Partials     9589     9603      +14     

Files with missing lines	Coverage Δ
lib/internal/test_runner/coverage.js	59.53% <0.00%> (-0.28%)	⬇️

... and 24 files with indirect coverage changes

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[inoway46]

If no section matches, could this fall through to ArrayPrototypeIndexOf(payload.sources, ...) and throw, since index maps have no top-level sources? Should we return undefined here? If so, a regression test for this case would also be helpful.