implement simulation of extern modules

crates/fayalite/tests/sim/expected/duplicate_names.txt

@@ -88,10 +88,14 @@ Simulation {
             ..
         },
     },
-    uninitialized_inputs: {},
-    io_targets: {},
-    made_initial_step: true,
-    needs_settle: false,
+    main_module: SimulationModuleState {
+        base_targets: [],
+        uninitialized_ios: {},
+        io_targets: {},
+        did_initial_settle: true,
+    },
+    extern_modules: [],
+    state_ready_to_run: false,
     trace_decls: TraceModule {
         name: "duplicate_names",
         children: [