fix: pdm export -f requirements exports package twice

[frostming]

Fixes #3123

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

All modified and coverable lines are covered by tests ✅

Project coverage is 85.65%. Comparing base (4271e42) to head (0ba2c77).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3126      +/-   ##
==========================================
+ Coverage   85.63%   85.65%   +0.01%     
==========================================
  Files         109      109              
  Lines       11032    11032              
  Branches     2408     2408              
==========================================
+ Hits         9447     9449       +2     
+ Misses       1093     1092       -1     
+ Partials      492      491       -1     

Flag	Coverage Δ
unittests	85.45% <100.00%> (+0.01%)	⬆️

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