Fix iterative cooks to keep track of CookAdditionalFiles and delete them when the package file that created them is invalidated

A licensee reports they are using the CookAdditionalFilesOverride function in UObject to add files to the project. This file is properly copied into the Staging area during Packaging, and is appropriately packaged to the final game. Then, if they remove the Example.uasset file, and do another Packaging, they see the MyNewFile.xyz is still present in the Staging area, even if Example.uasset is now deleted. That, in itself, is not a big deal. However, they also see the MyNewFile.xyz file is getting yet again packaged, even without its Example.uasset, with the new game that doesn't even have Example.uasset.

This is a bug with iterative cooking. All cooks from the editor after the first one are iterative.

Iterative cooking deletes packages that it detects are out of date, but the iterative deletion does not consider Additional files and so they remain.

Need to write a test case UObject that generates additional files...