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

[dmitryax]

Same as #14774, against failing release branch

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 91.37%. Comparing base (ede31f8) to head (91e8b93).
⚠️ Report is 1 commits behind head on prepare-release-prs/0.148.0.

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                       @@
##           prepare-release-prs/0.148.0   #14773      +/-   ##
===============================================================
- Coverage                        91.38%   91.37%   -0.01%     
===============================================================
  Files                              695      695              
  Lines                            44390    44400      +10     
===============================================================
+ Hits                             40567    40572       +5     
- Misses                            2689     2692       +3     
- Partials                          1134     1136       +2     

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