feat: Support for constraint files

[frostming]

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.

Close #2896

[codecov]

Codecov Report

Attention: Patch coverage is 85.10638% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.66%. Comparing base (8068a36) to head (0873981).
Report is 100 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/formats/requirements.py	72.00%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3003      +/-   ##
==========================================
+ Coverage   85.61%   85.66%   +0.04%     
==========================================
  Files         109      109              
  Lines       10869    10890      +21     
  Branches     2371     2376       +5     
==========================================
+ Hits         9306     9329      +23     
+ Misses       1076     1075       -1     
+ Partials      487      486       -1     

Flag	Coverage Δ
unittests	85.47% <85.10%> (+0.04%)	⬆️

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