`predictabilize_uuids` doesn't work as expected

Appreciate the awesome work you've done here!

We are using `Xcodeproj` in our `Podfile` ruby script to basically build up our Xcode project targets from scratch, because we whitelabel our app. On each install, we delete all targets, then we recreate them based on our own configuration list. We use `project.predictabilize_uuids`, however it gives a multitude of warnings of duplicated UUIDs (seems to have no impact aside from the warning), and even so, the UUIDs are not predicable, they all change on the next install regardless.

Can you give some pointers as to why the UUIDs are not stable? The entire project configuration itself is stable - most of it is deleted and recreated each time but the resulting content is stable aside from the UUIDs.

Start by reproducing the Podfile workflow that deletes and recreates targets, then inspect the predictabilize_uuids entry point and how it handles recreated project objects. Done means the stable project configuration produces stable UUIDs without duplicate-UUID warnings on subsequent installs.

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