Currently, our top-level `Cargo.toml` manifest represents a crate AND a workspace. This causes surprising behaviour (e.g. #2949) where we need to explicitly pass `--workpace` to every command to run it on the entire workspace and not just the meta crate.
My moving the meta crate into its own directory, the root manifest file is a virtual manifest only and thus, every `cargo` command will automatically default to running on the entire workspace.
On top of this, I personally find it easier to understand if workspace and crate manifests are not mixed.
Pull-Request: #3536.
