Use module scope instead of var store to generate idents in dbg desugar

Fix a bug in `dbg` expression desugaring by using the module scope to
generate unique identifiers instead of the variable store.

In the initial implementation of `dbg` expressions we used the
`VarStore` to generate unique identifiers for new variables created
during desugaring. We should have instead used the current module's
`Scope`, which handles identifiers within the module. Each scope has its
own incrementing variable count which is independent of the shared
variable store. The scope is used to generate new identifiers at other
points in canonicalization, such as when assigning a global identifier
to closures and `expect`s. It's possible that the identifier generated
for `dbg` could conflict with an identifier generated by the scope,
resulting in a confusing error.

crates/compiler/load/tests/helpers/mod.rs

@@ -161,6 +161,13 @@ pub fn can_expr_with<'a>(
     // ensure the Test module is accessible in our tests
     module_ids.get_or_insert(&PQModuleName::Unqualified("Test".into()));
 
+    let mut scope = Scope::new(
+        home,
+        "TestPath".into(),
+        IdentIds::default(),
+        Default::default(),
+    );
+
     // Desugar operators (convert them to Apply calls, taking into account
     // operator precedence and associativity rules), before doing other canonicalization.
     //
@@ -170,7 +177,7 @@ pub fn can_expr_with<'a>(
     // rules multiple times unnecessarily.
     let loc_expr = desugar::desugar_expr(
         arena,
-        &mut var_store,
+        &mut scope,
         &loc_expr,
         expr_str,
         &mut None,
@@ -178,13 +185,6 @@ pub fn can_expr_with<'a>(
         &mut Default::default(),
     );
 
-    let mut scope = Scope::new(
-        home,
-        "TestPath".into(),
-        IdentIds::default(),
-        Default::default(),
-    );
-
     let dep_idents = IdentIds::exposed_builtins(0);
     let mut env = Env::new(
         arena,