Make all dependencies workspace dependencies (#9333)
## Summary
This PR modifies our `Cargo.toml` files to use workspace dependencies
for _all_ dependencies, rather than the status quo of sporadically
trying to use workspace dependencies for those dependencies that are
used across multiple crates. I find the current situation more confusing
and harder to manage, since we have a mix of workspace and crate-local
dependencies, whereas this setup consistently uses the same approach for
all dependencies.
