packageurl.HandlerPackage returns the ecosystem and name the handlers store
a package URL under. The mirror and the inventory both use it, which
settles where they disagreed: distribution namespaces of apk, alpm, bsd,
deb and rpm are dropped, PyPI and NuGet names use purl's comparison form,
and Maven names are group:artifact as the handler stores them, where the
mirror used group/artifact.
A conda version is mirrored with every build anaconda.org lists for it,
each cached under the channel, name, version and subdir/filename the conda
download handler reads from its path. The channel comes from the namespace
or the channel qualifier; a package without one is refused, as retention
keys without one are. The subdir, build and type qualifiers select builds.
Which files a version has, which qualifiers select them and where the
handler caches them is now described per ecosystem, so PyPI and conda share
mirrorFiles.
Part of #414.
