Refactor project for a two-feature architecture

[danyeaw]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

Reorganize code to clearly separate:

1. Wheel plugin (~5%) - enables conda install <package from repodata
2. PyPI conversion (~95%) - provides conda pypi install/convert

Why is this needed?

Current flat structure is confusing. Hard to tell which code does what. Related to docs issue #231.

What should happen?

Move files into clear subdirectories:

conda_pypi/
├── wheel_plugin/ # whl.py, extract_whl.py
├── conversion/ # build.py, convert_tree.py, downloader.py, installer.py, translate.py
├── shared/ # main.py, python_paths.py, paths.py, utils.py
├── cli/ # keep as-is
├── dependencies/ # keep as-is
└── post_command/ # keep as-is

Additional Context

No response