feature request: hit detection

[rbtcollins]

Something that I use from time to time in the github actions cache is the ability to tell when a hit has occurred.

For instance, a cache keyed by a ${architecture}-Cargo.lock containing a built binary executable. Then when the dependencies are updated, the binary is rebuilt, but we don't call into cargo at all when they aren't. A separate cache is used to cache the contents of the dependencies, and so on to make builds faster if they are required.