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.
0f81fca8ec