fix: self-referencing extra dependencies doesn't work for local packages

[frostming]

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 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 85.32%. Comparing base (533be5f) to head (ef813c0).
Report is 2 commits behind head on main.

Files	Patch %	Lines
src/pdm/models/repositories.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2898   +/-   ##
=======================================
  Coverage   85.32%   85.32%           
=======================================
  Files         106      106           
  Lines       10596    10596           
  Branches     2307     2307           
=======================================
  Hits         9041     9041           
  Misses       1089     1089           
  Partials      466      466           

Flag	Coverage Δ
unittests	85.10% <50.00%> (ø)

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