Added ability to convert multiply-nested slices (#1239)

* Addressed feedback from previous PR
* Change missing entity aliases to be deterministic.
When converting slices, this helps produce the same results on multiple reconversions of the same data.
* Exposed the asset filter callback.
This allows the slice converter to specifically load nested slices as opposed to not loading *any* referenced assets.
* Added support for multiply-nested slice instance conversion.

Code/Tools/SerializeContextTools/SliceConverter.h

@@ -42,8 +42,6 @@ namespace AZ
             bool ConvertSliceFiles(Application& application);
 
         private:
-            using TemplateEntityIdPair = AZStd::pair<AzToolsFramework::Prefab::TemplateId, AZ::EntityId>;
-
             bool ConnectToAssetProcessor();
             void DisconnectFromAssetProcessor();
 
@@ -60,10 +58,32 @@ namespace AZ
             void SetParentEntity(const AZ::Entity& entity, const AZ::EntityId& parentId, bool onlySetIfInvalid);
             void PrintPrefab(AzToolsFramework::Prefab::TemplateId templateId);
             bool SavePrefab(AZ::IO::PathView outputPath, AzToolsFramework::Prefab::TemplateId templateId);
+            void ClearSliceAssetReferences(AZ::Entity* rootEntity);
+            void UpdateSliceEntityInstanceMappings(
+                const AZ::SliceComponent::EntityIdToEntityIdMap& sliceEntityIdMap,
+                const AZStd::string& currentInstanceAlias);
 
-            // Track all of the entity IDs created and the prefab entity aliases that map to them.  This mapping is used
-            // with nested slice conversion to remap parent entity IDs to the correct prefab entity IDs.
-            AZStd::unordered_map<TemplateEntityIdPair, AzToolsFramework::Prefab::EntityAlias> m_aliasIdMapper;
+            // When converting slice entities, especially for nested slices, we need to keep track of the original
+            // entity ID, the entity alias it uses in the prefab, and which template and nested instance path it maps to.
+            // As we encounter each instanced entity ID, we can look it up in this structure and use this to determine how to properly
+            // add it to the correct place in the hierarchy.
+            struct SliceEntityMappingInfo
+            {
+                SliceEntityMappingInfo(AzToolsFramework::Prefab::TemplateId templateId, AzToolsFramework::Prefab::EntityAlias entityAlias)
+                    : m_templateId(templateId)
+                    , m_entityAlias(entityAlias)
+                {
+                }
+
+                AzToolsFramework::Prefab::TemplateId m_templateId;
+                AzToolsFramework::Prefab::EntityAlias m_entityAlias;
+                AZStd::vector<AzToolsFramework::Prefab::InstanceAlias> m_nestedInstanceAliases;
+            };
+
+            // Track all of the entity IDs created and associate them with enough conversion information to know how to place the
+            // entities in the correct place in the prefab hierarchy and fix up parent entity ID mappings to work with the nested
+            // prefab schema.
+            AZStd::unordered_map<AZ::EntityId, SliceEntityMappingInfo> m_aliasIdMapper;
 
             // Track all of the created prefab template IDs on a slice conversion so that they can get removed at the end of the
             // conversion for that file.