RPM packages were cached under their bare filename. With upstream.rpm
pointing at a mirror root that serves several distributions, Rocky and
Alma both publish bash-5.1.8-9.el9.x86_64.rpm with different bytes, and
whichever was fetched first was served for both, so dnf failed the
checksum or signature check.
Use the path under the upstream as the cache filename, as the APK and
Arch handlers do. Packages cached before this change are no longer
found and are fetched again once; retention still matches them by
package name.
Packages are stored as pkg:rpm/{name}, without a vendor and with the name's case kept. Package rules may name a vendor the way PURLs usually do (pkg:rpm/fedora/curl); it's dropped. The docs row warns that Fedora and EPEL update repositories only keep the latest build.
The mirror can't fetch rpm packages, so there's no mirror test.
Refs #306
