fix: don't validate requirements that are not used

[frostming]

Fixes #3376

Signed-off-by: Frost Ming me@frostming.com

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 69.56522% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.43%. Comparing base (f0ad0c0) to head (e1bd7f9).
Report is 43 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/models/requirements.py	61.11%	3 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3377      +/-   ##
==========================================
+ Coverage   85.20%   85.43%   +0.23%     
==========================================
  Files         112      112              
  Lines       11492    11496       +4     
  Branches     2517     2521       +4     
==========================================
+ Hits         9792     9822      +30     
+ Misses       1173     1145      -28     
- Partials      527      529       +2     

Flag	Coverage Δ
unittests	84.49% <69.56%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.