A Debian package was cached under its repository and file name, but
fetched from its full path. Docker's apt repository has a pool under each
suite with the same file names, so a bookworm client could get the
bullseye build that was fetched first, and fail its hash check. Packages
are now cached under their path.
Conan joined revision, package reference, package revision and file name
with underscores, which revisions can contain, so a recipe file and a
package file could share a cache entry. The parts are now joined with
slashes.
Entries cached under the old names are still served, but only for the
URL they were fetched from, so warm caches keep working.
Packages are stored as pkg:deb/{name}, without a vendor and whichever configured archive they came from. Package rules may name a vendor the way PURLs usually do (pkg:deb/debian/curl, pkg:deb/ubuntu/curl); it's dropped, so a rule covers the package from every archive. The docs row warns that Debian and Ubuntu archives only keep the current version per suite.
The mirror can't fetch deb packages, so there's no mirror test.
Refs #306
