Run shadowed-variable analyses in deferred handlers (#5181)

## Summary

This PR extracts a bunch of complex logic from `add_binding`, instead
running the the shadowing rules in the deferred handler, thereby
decoupling the binding phase (during which we build up the semantic
model) from the analysis phase, and generally making `add_binding` much
more focused.

This was made possible by improving the semantic model to better handle
deletions -- previously, we'd "lose track" of bindings if they were
deleted, which made this kind of refactor impossible.

## Test Plan

We have good automated coverage for this, but I want to benchmark it
separately.

crates/ruff_python_semantic/src/scope.rs

@@ -126,6 +126,11 @@ impl<'a> Scope<'a> {
         })
     }
 
+    /// Returns the ID of the binding that the given binding shadows, if any.
+    pub fn shadowed_binding(&self, id: BindingId) -> Option<BindingId> {
+        self.shadowed_bindings.get(&id).copied()
+    }
+
     /// Adds a reference to a star import (e.g., `from sys import *`) to this scope.
     pub fn add_star_import(&mut self, import: StarImport<'a>) {
         self.star_imports.push(import);