refactor: move convert module to stable_mir

note that this commit delete `convert/error.rs`, we would use `SmirError::from_internal` instead.

**Unresolved questions:**
- There are still a few direct calls to rustc's internals scattered across `impl Stable`s, but most of them appear to be relatively stable, e.g., `mir::interpret::ConstAllocation::inner(self)` and `mir::syntax::SwitchTargets::otherwise(self)`.

compiler/rustc_smir/src/stable_mir/compiler_interface.rs

@@ -8,6 +8,7 @@ use std::cell::Cell;
 use rustc_hir::def::DefKind;
 use rustc_smir::{Bridge, SmirContainer, Tables};
 use stable_mir::abi::{FnAbi, Layout, LayoutShape, ReprOptions};
+use stable_mir::convert::{RustcInternal, Stable};
 use stable_mir::crate_def::Attribute;
 use stable_mir::mir::alloc::{AllocId, GlobalAlloc};
 use stable_mir::mir::mono::{Instance, InstanceDef, StaticDef};