Enable storage retention for conda

[pinguinfuss]

The handler stores packages as {channel}/{name}, which purl writes as pkg:conda/conda-forge%2Fnumpy. Package rules name the channel as the namespace (pkg:conda/conda-forge/numpy) and get mapped to that form, with the package name lowercased the way conda treats it. A key without a channel or with an empty part could never match and is refused.

The mirror can't fetch conda packages today, so there's no mirror test.

Refs #306

[andrew]

condaRetentionKey accepts any number of channel segments, and the comment says a label channel such as conda-forge/label/rc keeps all its segments. The handler can't store one though: downloads are routed as GET /{channel}/{arch}/{filename}, so {channel} is always a single path segment and the stored name is always {channel}/{name}. A rule like pkg:conda/conda-forge/label/rc/numpy therefore passes validation and matches nothing, which is the case the key mapping refuses for keys without a channel.

Please require exactly two segments (channel and name), drop the label channel sentence from the comment, and flip the conda-forge/label/rc/numpy case in TestCondaRetentionKey to refused. This will also need rebasing for the docs/configuration.md table once the neighbouring retention PRs are in.

[pinguinfuss]

Fixed in c58cdae: condaRetentionKey now needs exactly a channel and a name, the label channel sentence is gone, and conda-forge/label/rc/numpy is refused in TestCondaRetentionKey (also in its %2F form). Rebased onto main for the docs table.