[cmd/mdatagen] Localize same-repo schema refs using import root path

[dmitryax]

This change make mdatagen to rewrite same-repo schema references to absolute local refs during schema generation and resolution.

Instead of keeping refs like go.opentelemetry.io/collector/filter.config as versioned external references, mdatagen now converts them to /filter.config when the ref belongs to the current repo import root.

This avoids generator failures in release/version-bump PRs where mdatagen would otherwise try to resolve schemas from a not-yet-published version tag or .schemas/ cache. See https://github.com/open-telemetry/opentelemetry-collector/actions/runs/23166843771/job/67309001982?pr=14772

[codecov]

Codecov Report

❌ Patch coverage is 92.59259% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.38%. Comparing base (7cd8f58) to head (cf88171).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/mdatagen/internal/command.go	91.30%	0 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (92.59%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14774      +/-   ##
==========================================
- Coverage   91.40%   91.38%   -0.02%     
==========================================
  Files         695      695              
  Lines       44390    44400      +10     
==========================================
+ Hits        40573    40575       +2     
- Misses       2685     2690       +5     
- Partials     1132     1135       +3     

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[codspeed-hq]

Merging this PR will improve performance by 30.43%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 6 untouched benchmarks
⏩ 76 skipped benchmarks¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	BenchmarkTraceSizeSpanCount	60 ns	46 ns	+30.43%

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_{Comparing dmitryax:fix-mdatagen-refs (cf88171) with main (7cd8f58)}

Footnotes

1. 76 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩