Virtual repos cover rpm, deb, npm and others but not alpine, so a local alpine repo and github_alpine remotes cannot be served as one apk repository.
- merge members' `<dir>/APKINDEX.tar.gz` per directory; member order wins duplicate (P, V, A); a member 404 is skipped
- 302 `<dir>/<P>-<V>.apk` downloads to the member whose index listed the package
- serve the merged index unsigned, like local and github_alpine indexes
- cache merges per directory on the shared virtual cache; member errors fail with 502
- add docs/alpine/virtual.md, unit tests and an apk e2e with abuild-built packages
Requires unkin/artifactapi#163.
Reviewed-on: #164
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>
