Minimise recipe.json when cargo chef prepare is called with a --bin flag

[LukeMathWalker]

cargo-chef slims down the workspace members to just the specified binary when cargo chef prepare is invoked with a --bin flag.
At the moment (see #207) we still keep around ALL other local manifests, in case the binary depends on another one of the local crates via a path dependency.
To maximise cache reuse, we could be more sophisticated: only include in the recipe.json file the manifests of the local crates that are in the dependency closure of the specified binary target.

[markdingram]

I think this would follow on nicely from #169

An earlier version had a "collect_local_packages" that walked down from the workspace_packages collecting all local manifests
585a0f5

Realised that wasn't necessary for #169, but it could serve as the basis for this