Parallelize downloading project deps and blobs from registry

[kylewlacy]

This PR improves how project downloads are handled from the registry:

• Project dependencies are now downloaded in parallel. This might turn out to be problematic in the future because this download is recursive but has no semaphore to cap the total number of recursive downloads. This should suffice for a while (and I'm not confident enough in how to solve this in a way that wouldn't deadlock)
• Project module blobs are now downloaded with the brioche_core::registry::fetch_blobs function, which will both download blobs in parallel and avoid downloading blobs that have been downloaded before