Conda and Mamba both support having multiple cache directories. Rattler currently only supports a single directory. The "layered" cache can have multiple readable directories and only one writable directory. Conda/Mamba tries to write a magical file to all these directories at startup to determine which one is writable.

Rattler should also facilitate "layered" package caches. We could introduce a trait for PackageCache that is implemented for both the current implementation as well as a layered version.