Source archives are stored under the registry identity, pkg:generic/swift-registry/{scope}.{name}, lowercased. Package rules write the scope as the namespace (pkg:swift/apple/swift-nio) or use the stored form, in any case. A source repository key like pkg:swift/github.com/apple/swift-nio doesn't name a registry identity and is refused.
The mirror can't fetch swift packages, so there's no mirror test.
Refs #306
