Add docstrings and comments to complex functions missing documentation

[LalatenduMohanty]

Summary

Several functions across src/fromager/ have complex logic but lack docstrings or inline comments, making it harder for new contributors to understand the codebase.

Docstrings needed (12 functions) — complex logic with multiple code paths or non-obvious algorithms:

• find_sdist, find_wheel, find_source_dir (finders.py) — multiple naming fallback strategies
• get_project_from_pypi (resolver.py) — multi-stage filtering pipeline
• _resolve_version_from_git_url (bootstrapper.py) — git URL parsing with ref extraction and clone caching
• add_extra_metadata_to_wheels, extract_info_from_wheel_file (wheels.py) — multi-step pipeline, normalization edge case
• download_url, prepare_source (sources.py) — retry/temp file handling, polymorphic plugin returns
• get_build_backend_hook_caller (dependencies.py) — 3-source env var merge
• _update_build_requires (pyproject.py) — normalized-name dedup algorithm
• run (external_commands.py) — expand existing minimal docstring