Add FetchCode support for github

[TG1999]

PURL: pkg:github/expressjs/express@4.17.1

Download URL:
https://github.com/expressjs/express/archive/refs/tags/4.17.1.zip

[TG1999]

This is done in

https://github.com/package-url/packageurl-python/blob/main/src/packageurl/contrib/purl2url.py#L438

Commit:
package-url/packageurl-python@2b7e68a