Rollup merge of #140532 - celinval:chores-smir-ra, r=oli-obk

Fix RustAnalyzer discovery of rustc's `stable_mir` crate

This fixes issues with RustAnalyzer not finding `stable_mir` crate since RA discovery traverses the dependency graph of `rustc_driver` crate.

This change also aligns with the long term architecture plan for these crates, since we are moving towards having stable_mir depend on rustc_smir and not the other way around. See [this doc](https://hackmd.io/jBRkZLqAQL2EVgwIIeNMHg) for more details.

I believe a similar function will come handy eventually for `stable_mir` users, but I'm keeping it as part of `rustc_internal` since its current format initializes the StableMir context and requires `TyCtxt`.

Finally, I added the `rustc_internal` module re-export under a feature since the APIs from this module shall not be stabilized.

Author: Zalathar