Fix race condition when trying to fetch the same project from the registry more than once

[kylewlacy]

Closes #128

This PR updates the fetch_project_from_registry function to use a mutex to ensure that the same project doesn't get cloned more than once at a time. This works by using a BTreeMap (wrapped in a Mutex), where each key-value pair is a project hash and another mutex

As far as implementation, there are a few other places in the code where we have a similar problem of wanting to ensure that a piece of code only runs once for a particular key: baking recipes, and creating outputs. For baking, we use a map that contains channels, which allows parallel calls to get the result immediately without doing more work. For creating outputs, we use a lazier option of using a single mutex to guard the whole section-- it's not (currently) fine-grained. This new implementation is kind of like a mix between these two other implementations, where we do have fine-grained locking, but each invocation ends up doing a bit of work to read the filesystem to figure out where the output goes

One downside with this implementation is that the mutex map uses Arcs, and it never gets keys removed. We could switch to using Weak values then add a "remove" phase, but I thought it'd be unlikely that enough keys could ever be added to the map to become an issue