Rewrote PyPI mirror endpoints due to change in the way PyPI serves package URLs

[tieneupin]

• Simple index is now located under '/pypi/index/'
• Package URL paths now mirror Python package repo's URL structure ('https://files.pythonhosted.org/packages/path/to/python/package' --> '{base_url}/pypi/packages/path/to/python/package')

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 13 lines in your changes missing coverage. Please review.

Project coverage is 31.85%. Comparing base (1c08889) to head (47b01b1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #613      +/-   ##
==========================================
+ Coverage   31.78%   31.85%   +0.07%     
==========================================
  Files          90       90              
  Lines       10750    10726      -24     
  Branches     1423     1416       -7     
==========================================
  Hits         3417     3417              
+ Misses       7218     7193      -25     
- Partials      115      116       +1     

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