Prefab Serialization | Remove "Source" parameter from prefab source file (#1387)

* When loading a template to memory, store the current Source (potentially overriding what was stored to disk)

* Before storing the prefab disk, remove the Source parameter from the Dom (as it's no longer necessary to save it).

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabLoader.cpp

@@ -151,6 +151,10 @@ namespace AzToolsFramework
                 return InvalidTemplateId;
             }
 
+            // Add or replace the Source parameter in the dom
+            PrefabDomPath sourcePath = PrefabDomPath((AZStd::string("/") + PrefabDomUtils::SourceName).c_str());
+            sourcePath.Set(readPrefabFileResult.GetValue(), relativePath.Native().c_str());
+
             // Create new Template with the Prefab DOM.
             TemplateId newTemplateId = m_prefabSystemComponentInterface->AddTemplate(relativePath, readPrefabFileResult.TakeValue());
             if (newTemplateId == InvalidTemplateId)

Code/Framework/AzToolsFramework/AzToolsFramework/Prefab/Template/Template.cpp

@@ -207,6 +207,10 @@ namespace AzToolsFramework
                 instanceValue->CopyFrom(linkDom, m_prefabDom.GetAllocator());
             }
 
+            // Remove Source parameter from the dom. It will be added on file load, and should not be stored to disk.
+            PrefabDomPath sourcePath = PrefabDomPath((AZStd::string("/") + PrefabDomUtils::SourceName).c_str());
+            sourcePath.Erase(output);
+
             return true;
         }