Draft: feat: use github metadata if no pypi release found

The current implementation always uses PyPI to find out metadata for the package, even if a Github link has been provided. This is problematic for packages that only have their source available on Github, but without any release available on PyPI.

The code was refactored to put some functions in common, and a new function has been added to extract metadata from the Github repository if no PyPI release is found, as a fallback.