In the cache, we need to be able to separately store foo 1.0.0 and foo 2.0.0 (for example), but when using git cookbooks, we don't have any guarantee that there won't be multiple commits with the same version number, so we need to support alternate directory names, e.g. `foo-SHASUM`. At the end of the process, we want to produce a data file that gives us the necessary information to upload the cookbooks without data from any external source, so we need to know where we expect to find the local copy. From the presence of the `cache_key` we can infer that we're using a cached copy, so we know to look in the cache. Since we're exposing this functionality to users to create their own workflows, I don't see any reason to constrain the cache_key value to a particular format.