Change mod import path to github or valid path

[wojas]

Change the powerdns.com/platform/lightningstream import path to github.com/PowerDNS/lightningstream so that go install and https://pkg.go.dev/github.com/PowerDNS/lightningstream work correctly.

The alternative would be to add these url endpoints with special html meta tags on our website and make sure it always works, which is probably not worth the trouble.