fix: support relative paths in build requires

[frostming]

Pull Request Check List

• 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.

Close #1560

[codecov-commenter]

Codecov Report

Base: 85.31% // Head: 85.22% // Decreases project coverage by -0.08% ⚠️

Coverage data is based on head (30f16f0) compared to base (67ee5da).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1561      +/-   ##
==========================================
- Coverage   85.31%   85.22%   -0.09%     
==========================================
  Files          91       91              
  Lines        8062     8063       +1     
  Branches     1910     1911       +1     
==========================================
- Hits         6878     6872       -6     
- Misses        794      798       +4     
- Partials      390      393       +3     

Flag	Coverage Δ
unittests	85.05% <0.00%> (-0.09%)	⬇️

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

Impacted Files	Coverage Δ
src/pdm/installers/core.py	100.00% <ø> (ø)
src/pdm/models/repositories.py	79.21% <0.00%> (-1.50%)	⬇️
src/pdm/models/caches.py	87.71% <0.00%> (-1.17%)	⬇️
src/pdm/termui.py	89.85% <0.00%> (-0.73%)	⬇️

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