Add support for AP-compliant relative paths (#998)

The method "PrefabLoader::GetRelativePathToProject" has been changed to "PrefabLoader::GenerateRelativePath", and reworked to get a correct relative path.  GetFullPath has also been modified to get correct relative paths too.  This requires an Asset Processor connection - if one isn't available (like during unit tests), the methods have fallback logic to produce project-relative paths.

With this change, SliceConverter can't use SaveTemplate() to save the file any more, because GetFullPath now expects to find an existing path, which doesn't work for not-yet-created files.  Instead, it now has to use the same technique as the Editor and call SaveTemplateToString then save the string out as a file.

Code/Tools/SerializeContextTools/SliceConverter.h

@@ -56,7 +56,7 @@ namespace AZ
                 AZ::SliceComponent::SliceInstance& instance, AZ::Data::Asset<AZ::SliceAsset>& sliceAsset,
                 AzToolsFramework::Prefab::TemplateReference nestedTemplate, AzToolsFramework::Prefab::Instance* topLevelInstance);
             static void PrintPrefab(AzToolsFramework::Prefab::TemplateId templateId);
-            static bool SavePrefab(AzToolsFramework::Prefab::TemplateId templateId);
+            static bool SavePrefab(AZ::IO::PathView outputPath, AzToolsFramework::Prefab::TemplateId templateId);
         };
     } // namespace SerializeContextTools
 } // namespace AZ