fix: feat: add back candidate.build() for backward-compatibility

[frostming]

Fixes #2747

Signed-off-by: Frost Ming [email protected]

Pull Request Checklist

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 85.06%. Comparing base (397d9b0) to head (486cd53).

Files	Patch %	Lines
src/pdm/models/candidates.py	72.72%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2750      +/-   ##
==========================================
- Coverage   85.06%   85.06%   -0.01%     
==========================================
  Files         106      106              
  Lines       10556    10569      +13     
  Branches     2293     2297       +4     
==========================================
+ Hits         8979     8990      +11     
  Misses       1103     1103              
- Partials      474      476       +2     

Flag	Coverage Δ
unittests	84.84% <76.92%> (-0.01%)	⬇️

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