Conda download caching keyed artifacts on channel, name, version and
filename only, but the same filename can hold different bytes in each
subdir (python_abi-3.11-4_cp311.conda in linux-64 and osx-arm64, for
example). A client on one platform could be served another platform's
package. The subdir is now part of the cached filename, so each one gets
its own entry. Entries cached under the old key are no longer used and
are re-fetched on first request.
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
