fix: export editable local dependencies with relative paths

[tunglambk]

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.

Fixes #3294.

Editable local dependencies are exported using their relative project path instead of expanding file:///${PROJECT_ROOT} into an absolute file:// URL. This keeps generated requirements files portable across machines and checkout locations.

The change is limited to editable FileRequirement entries that have a local path. VCS, remote URLs, and non-editable requirements keep their existing formatting.

This supersedes #3853, whose CI regression-test fixture was corrected here.

Verification

• .venv/bin/python -m pytest -q tests/test_formats.py tests/test_requirements_export_relative.py (62 passed)
• ruff check src/pdm/formats/requirements.py tests/test_requirements_export_relative.py
• ruff format --check src/pdm/formats/requirements.py tests/test_requirements_export_relative.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.51%. Comparing base (394fe28) to head (84104ea).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3856   +/-   ##
=======================================
  Coverage   88.51%   88.51%           
=======================================
  Files         121      121           
  Lines       13278    13281    +3     
  Branches     2256     2257    +1     
=======================================
+ Hits        11753    11756    +3     
  Misses        960      960           
  Partials      565      565           

Flag	Coverage Δ
unittests	88.40% <100.00%> (+<0.01%)	⬆️

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