chore: change project structure

[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 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.14%. Comparing base (b448f41) to head (a5c1a8a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/pdm/__version__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3155      +/-   ##
==========================================
+ Coverage   85.68%   86.14%   +0.45%     
==========================================
  Files         109      107       -2     
  Lines       11049    10730     -319     
  Branches     2415     2294     -121     
==========================================
- Hits         9467     9243     -224     
+ Misses       1091     1022      -69     
+ Partials      491      465      -26     

Flag	Coverage Δ
unittests	85.93% <66.66%> (+0.45%)	⬆️

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