When caching is too fine-grained, an excessive number of cache
requests can result in HTTP 429 errors due to rate limiting.
By caching all artifacts of a particular type in a single entry
we hope to mitigate this, at the expense of some reduction in
cache space optimization.
This change also adds caching for all dependency jars, as well as
instrumented jars in the 'caches/jars-X' directory.