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.
