Rollup merge of #143524 - makai410:smir-move-back, r=oli-obk

Move `stable_mir` back to its own crate

We've finished the refactoring, so it's time to move `stable_mir` back to its own crate.

This PR leaves an empty `rustc_internal` module with a `#[deprecated]` attribute in `rustc_smir` to let users know we just moved it to `stable_mir`.

Author: jieyouxu