Remove the monolithic utils::fetch module (HTTP download, git clone,
checksum, extraction) and utils::fetch_plugin module from the baker.
Introduce a ResourceRegistry type to centrally track pipeline
resources by name and path.
The actual fetching logic is now delegated to the workshop-getterurl
crate, which gains optional md5 and sha1 hash support behind default
features. Also flatten the builtin module in workshop-getterurl.