Clean up our dependency tree with cargo-hack

[smalis-msft]

We currently have a cargo-machete-based check in xtask fmt that ensures that crates don't include dependencies that they don't use at all. While this check is fast, it isn't perfect.
 
Running a command like RUSTFLAGS=-Dunused-crate-dependencies cargo hack --workspace --feature-powerset check indicates a lot of cases where we are building dependencies that aren't actually being used for a particular configuration. For example: something used only by tests that is in dependencies instead of dev-dependencies, or something used only on one platform (windows vs linux) that is included globally, or something only needed when a feature is enabled that is always included, etc. None of these can be caught by our current cargo-machete-based approach. Cleaning all these up would be very tedious but would likely get us a little bit of compile time improvement.