Missing escaping when creating import of existing Go module (#5767)

### Description
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See test for what this fixes: without this fix we end up with an import
on the consumer side that says `import Math ".../Math"` when the correct
name is `Math_`.

### How has this been tested?
Augmented the `timesTwo.dfy` separate compilation test.

<small>By submitting this pull request, I confirm that my contribution
is made under the terms of the [MIT
license](https://github.com/dafny-lang/dafny/blob/master/LICENSE.txt).</small>

---------

Co-authored-by: Mikael Mayer <[email protected]>

Source/DafnyCore/Backends/GoLang/GoCodeGenerator.cs

@@ -253,7 +253,8 @@ protected override void DependOnModule(Program program, ModuleDefinition module,
         }
       }
 
-      var import = CreateImport(module.GetCompileName(Options), module.IsDefaultModule, externModule, libraryName);
+      var publicModuleName = PublicModuleIdProtect(module.GetCompileName(Options));
+      var import = CreateImport(publicModuleName, module.IsDefaultModule, externModule, libraryName);
       import.Path = goModuleName + import.Path;
       AddImport(import);
     }

Source/IntegrationTests/TestFiles/LitTests/LitTest/comp/separate-compilation/Inputs/producer/timesTwo.dfy

@@ -23,3 +23,10 @@ module {:options "--function-syntax:4"} CoolLibraryName {
 
   }
 }
+
+// Ensuring reserved names in some languages are escaped consistently across languages
+module Math {
+  function Add(x: int, y: int): int {
+    x + y
+  }
+}

Source/IntegrationTests/TestFiles/LitTests/LitTest/comp/separate-compilation/usesTimesTwo.dfy

@@ -50,12 +50,16 @@ include "Inputs/producer/timesTwo.dfy"
 module ConsumerModule {
 
   import opened CoolLibraryName.LibraryModule
+  import Math
 
   method Main() {
     var n := 21;
     var TwoN := TimesTwo(n);
     print "Two times ", n, " is ", TwoN, "\n";
 
+    var z := Math.Add(20, 22);
+    print "20 plus 22 is ", z, "\n";
+
     // Need to actually execute the use of nat's type descriptor
     // to ensure it works on dynamic language targets.
     var aNat := PickANat();

Source/IntegrationTests/TestFiles/LitTests/LitTest/comp/separate-compilation/usesTimesTwo.dfy.expect

@@ -1,4 +1,8 @@
 Two times 21 is 42
+20 plus 22 is 42
 Two times 21 is 42
+20 plus 22 is 42
 Two times 21 is 42
+20 plus 22 is 42
 Two times 21 is 42
+20 plus 22 is 42