Packages are stored as pkg:apk/alpine/{name} whichever configured repository they came from, so a package rule covers the package in all of them. Rules may leave out the alpine namespace (pkg:apk/curl); another namespace could never match and is refused. The docs row warns that Alpine mirrors only keep the current build, so an evicted older one can't come back.
The mirror can't fetch apk packages, so there's no mirror test.
Refs #306
