x/pkgsite: support launchpad.net in source.ModuleInfo

[julieqiu]

Add support for getting source from launchpad.net.

https://github.com/golang/pkgsite/blob/4ea5ad9/internal/source/source.go#L273

Related: #39621

[amarjeetanandsingh]

@julieqiu
May I give it a try?

[jba]

@amarjeetanandsingh If you're still interested, we'd love to have your help!

[amarjeetanandsingh]

Thanks @jba
I will start looking into it.