fix: sourcemap sources path relative to output file dirname

[underfin]

Description

[netlify]

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[netlify]

✅ Deploy Preview for rolldown-rs canceled.

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[codspeed-hq]

CodSpeed Performance Report

Merging #758 will not alter performance

_{Comparing fix-sourcemap-path (757c18b) with main (ee3739d)}

Summary

✅ 6 untouched benchmarks

[codecov]

Codecov Report

Attention: Patch coverage is 85.00000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 79.79%. Comparing base (ee3739d) to head (757c18b).
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/rolldown_sourcemap/src/lib.rs	57.14%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #758   +/-   ##
=======================================
  Coverage   79.78%   79.79%           
=======================================
  Files         127      127           
  Lines        6569     6582   +13     
=======================================
+ Hits         5241     5252   +11     
- Misses       1328     1330    +2     

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