Rollup merge of #104953 - jyn514:fewer-submodule-updates, r=Mark-Simu…

…lacrum

Ensure required submodules at the same time as updating existing submodules

In practice, this would always happen at the same time, but putting them next to each other makes that more obvious and ensures it doesn't change in the future. It also avoids the difference affecting `cargo metadata` somehow.

This is based on rust-lang/rust#104952 for convenience to avoid merge conflicts, but doesn't depend on that PR.