cargo metadata output should be reproducible

[matklad]

Inside rust-analyzer repository:

λ cargo metadata > a.json
λ cargo metadata > b.json
λ diff --brief  a.json b.json
Files a.json and b.json differ
λ cargo --version
cargo 1.44.1 (88ba85757 2020-06-11)

I think it generally would be better if this was reproducible. In particular, in rust-analyzer we'd love to avoid reloading the workspace if "nothing changed", and non-reproducibility requires us to additionally sort things.