refactor: lazy import to reduce the startup time

[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.

Before change:

$ time pdm install --help
0.16s user 0.04s system 88% cpu 0.228 total

After change

$ time pdm install --help
0.12s user 0.04s system 92% cpu 0.168 total

[codecov-commenter]

Codecov Report

Patch coverage: 90.00% and project coverage change: -0.05 ⚠️

Comparison is base (44480a6) 85.19% compared to head (e30eadd) 85.15%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1929      +/-   ##
==========================================
- Coverage   85.19%   85.15%   -0.05%     
==========================================
  Files          99       99              
  Lines        9241     9243       +2     
  Branches     2010     2010              
==========================================
- Hits         7873     7871       -2     
- Misses        934      938       +4     
  Partials      434      434              

Flag	Coverage Δ
unittests	84.96% <90.00%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/pdm/cli/commands/publish/repository.py	53.04% <57.14%> (-1.83%)	⬇️
src/pdm/cli/utils.py	83.57% <66.66%> (ø)
src/pdm/environments/base.py	70.07% <75.00%> (-0.30%)	⬇️
src/pdm/models/repositories.py	73.86% <83.33%> (+0.18%)	⬆️
src/pdm/cli/actions.py	80.60% <100.00%> (ø)
src/pdm/cli/commands/publish/__init__.py	84.70% <100.00%> (ø)
src/pdm/cli/commands/publish/package.py	75.93% <100.00%> (+0.36%)	⬆️
src/pdm/cli/options.py	94.44% <100.00%> (ø)
src/pdm/core.py	93.52% <100.00%> (-0.05%)	⬇️
src/pdm/formats/flit.py	73.03% <100.00%> (-0.88%)	⬇️
... and 9 more

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