Fix alternative module reference case

Source/DafnyCore/Backends/GoLang/GoCodeGenerator.cs

@@ -1854,10 +1854,10 @@ protected override string TypeName_Companion(Type type, ConcreteSyntaxTree wr, I
       // XXX This duplicates some of the logic in UserDefinedTypeName, but if we
       // don't do it here, we end up passing the name of the module to
       // FormatCompanionName, which doesn't help anyone
-      if (type is UserDefinedType udt && udt.ResolvedClass != null && IsExternMemberOfExternModule(member, udt.ResolvedClass)) {
+      if (type is UserDefinedType { ResolvedClass: not null } udt && IsExternMemberOfExternModule(member, udt.ResolvedClass)) {
         // omit the default class name ("_default") in extern modules, when the class is used to qualify an extern member
         Contract.Assert(!udt.ResolvedClass.EnclosingModuleDefinition.IsDefaultModule);  // default module is not marked ":extern"
-        return IdProtect(udt.ResolvedClass.EnclosingModuleDefinition.GetCompileName(Options));
+        return IdProtect(ModuleImports[udt.ResolvedClass.EnclosingModuleDefinition].Name);
       }
       return TypeName_Related(FormatCompanionName, type, wr, tok, member);
     }
@@ -2729,7 +2729,6 @@ private string UserDefinedTypeName(TopLevelDecl cl, bool full, MemberDecl/*?*/ m
           // Don't use IdName since that'll capitalize, which is unhelpful for
           // built-in types
           return qual + (qual == "" ? "" : ".") + cl.GetCompileName(Options);
-
         } else if (!full || cl.EnclosingModuleDefinition.TryToAvoidName || this.ModuleName == enclosingModuleDefinitionId) {
           return IdName(cl);
         } else {

Source/DafnyCore/Backends/SinglePassCodeGenerator/SinglePassCodeGenerator.cs

@@ -12,6 +12,7 @@
 using System.IO;
 using System.Diagnostics.Contracts;
 using DafnyCore;
+using DafnyCore.Options;
 using JetBrains.Annotations;
 using Microsoft.BaseTypes;
 using static Microsoft.Dafny.GeneratorErrors;