Auto merge of #4938 - ehuss:ws-meta, r=alexcrichton

Auto merge of #4938 - ehuss:ws-meta, r=alexcrichton

Add workspace root to metadata command.

Fixes #4933

@alexcrichton, you mentioned using `"workspace_manifest"`, but the `Workspace.root` function already strips off `Cargo.toml`.  It would be easy to append it back, though I'm uncertain if that's really necessary since I think for most use cases it will just need to be stripped off again.  Also, I feel like it might be confusing for non-workspace packages since `workspace_manifest` would be the same as the package `manifest_path` (and in that case it isn't really a workspace manifest).  I can easily change it, just let me know.