To save space, future versions of Gradle are likely to delete the downloaded distribution
after extracting it. See gradle/gradle#3605 and gradle/gradle#19495.
To cater for this we will now save/restore the extracted distribution rather than the
downloaded zip file.
Instead of using a fallback strategy to locate a configuration-cache entry
based on the current job and git SHA, these entries are now keyed based on their
file content with the keys persisted in the primary Gradle User Home entry.
This removes the chance of having a configuration-cache entry restored that is
incompatible with the restored Gradle User Home state, and makes the logic easier
to understand.
This change involved a fairly major refactor, with the CacheEntryExtractor being
split out from the primary cache implementation, and adding a separate extractor
implementation for configuration-cache.
