fix: install --plugins cannot install self package

[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-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 🎉

Comparison is base (51c6ccb) 84.56% compared to head (4dc6eff) 84.63%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2062      +/-   ##
==========================================
+ Coverage   84.56%   84.63%   +0.07%     
==========================================
  Files         100      100              
  Lines        9607     9612       +5     
  Branches     2101     2103       +2     
==========================================
+ Hits         8124     8135      +11     
+ Misses       1028     1022       -6     
  Partials      455      455              

Flag	Coverage Δ
unittests	84.41% <100.00%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
src/pdm/installers/core.py	100.00% <ø> (ø)
src/pdm/cli/commands/install.py	89.36% <100.00%> (+0.23%)	⬆️
src/pdm/cli/templates/__init__.py	88.13% <100.00%> (+0.20%)	⬆️
src/pdm/installers/synchronizers.py	85.66% <100.00%> (+0.78%)	⬆️
src/pdm/resolver/core.py	92.30% <100.00%> (ø)

... and 1 file with indirect coverage changes

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