Change rustc_driver dependency on stable_mir crate

This fixes issues with RustAnalyzer not finding stable_mir crate.
It is also part of 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.

I believe this is an utility function that will come handy eventually
for stable_mir users, but I'm keeping it as part of rustc_internal since
it initializes the StableMir context and requires `TyCtxt`.

Finally, I added the rustc_internal crate under a feature since the APIs
from this module shall not be stabilized.

compiler/stable_mir/src/lib.rs

@@ -4,4 +4,8 @@
 //! This is a transitional measure as described in [PR #139319](https://github.com/rust-lang/rust/pull/139319).
 //! Once the refactoring is complete, the `stable_mir` implementation will be moved back here.
 
+/// Export the rustc_internal APIs. Note that this module has no stability
+/// guarantees and it is not taken into account for semver.
+#[cfg(feature = "rustc_internal")]
+pub use rustc_smir::rustc_internal;
 pub use rustc_smir::stable_mir::*;