Namespace cached files

Currently if I fetch a `master.tar.gz` from Github this would be saved in Starforge's cache as `master.tar.gz` and reused for any other attempt to fetch a `master.tar.gz`. Either some sort of randomized/hashed naming should be used, or at least have subdirectories of the cache directory namespaced to the package name.

Start by tracing how fetched archives are named and stored in Starforge's cache. Verify the behavior with two packages that use the same archive filename, then make sure their cached files remain distinct and confirm that repeated fetches for the same package still reuse the cache.

Written by the indexing model from the issue text.