Unavailable URLs returned by purl2url on GitHub PURLs

[tdruez]

>>> from packageurl.contrib import purl2url
>>> purl2url.purl2url('pkg:github/package-url/packageurl-python@0.9.1')
'https://github.com/package-url/packageurl-python/tree/0.9.1'

The returned URL https://github.com/package-url/packageurl-python/tree/0.9.1 leads to a 404.
The proper URL is actually https://github.com/package-url/packageurl-python/tree/v0.9.1 with the "v" prefix.

Note that the "v" prefix in a tag name is a convention and optional.

[scovetta]

Hi @tdruez I think the issue here is that the "version" for GitHub is taken to mean a tag, so your purl should actually contain the "v" prefix here, specifically because the packageurl-python project uses "v" as a prefix for tags.

>>> purl2url.purl2url('pkg:github/package-url/packageurl-python@v0.9.1')
'https://github.com/package-url/packageurl-python/tree/v0.9.1'

Other projects, like Python-Pillow, use just the raw version number, so this works as expected:

>>> purl2url.purl2url('pkg:github/python-pillow/Pillow@7.2.0')
'https://github.com/python-pillow/pillow/tree/7.2.0'

[tdruez]

The version prefix is now kept as a version_prefix qualifier in url2purl. This allows to generate valid URLs in purl2url.
We choose this approach instead of keeping the prefix in the @ version for maximum compatibility across existing PURLs.