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/Framework/AzToolsFramework/AzToolsFramework/Prefab/PrefabSystemComponent.cpp

@@ -95,7 +95,7 @@ namespace AzToolsFramework
             const AZStd::vector<AZ::Entity*>& entities, AZStd::vector<AZStd::unique_ptr<Instance>>&& instancesToConsume,
             AZ::IO::PathView filePath, AZStd::unique_ptr<AZ::Entity> containerEntity, bool shouldCreateLinks)
         {
-            AZ::IO::Path relativeFilePath = m_prefabLoader.GetRelativePathToProject(filePath);
+            AZ::IO::Path relativeFilePath = m_prefabLoader.GenerateRelativePath(filePath);
             if (GetTemplateIdFromFilePath(relativeFilePath) != InvalidTemplateId)
             {
                 AZ_Error("Prefab", false,