Support non-GitHub forges

[DivineDominion]

GitHub is still the de facto standard to host and share projects. Nevertheless, people have been starting to migrate.

If SPI could allow other forges, that would be a step forward to a more open ecosystem and encourage moving code and parts of the Swift OSS community elsewhere.

Noticed this limitation in SwiftPackageIndex/PackageList#11828

[daveverwer]

Both @finestructure and I agree with this being an important issue and it's something we have discussed several times over the years. There’s even a very old issue related to it, #860. 😬

That said, supporting additional git hosting providers isn’t something we're able to do in the short to medium term. We appreciate the motivation behind wanting a more open ecosystem, but this isn’t something we can tackle right now.