Avoid rebuild old FindTrackedObjectsInJson into json CalculateDiff

[PolEpie]

Pull Request

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Summary / Motivation & Context

This is part of a big finding on my #11630 issue
So i saw that FindTrackedObjectsInJson was called twice and did it with old / new, and i was like hmm, couldnt that being cached ?

Implementation Details

So my pr is simple, into PrefabCacheScene.Load, we calculate the Json.BuildTrackedObjects directly, and use it on FindTrackedObjectsInJson instead of recalculate it.
Anyway once a prefab is saved it's gonna recall load and update it

Checklist

• Code follows existing style and conventions
• No unnecessary formatting or unrelated changes
• Public APIs are documented (if applicable)
• Unit tests added where applicable and all passing
• I’m okay with this PR being rejected or requested to change 🙂

[lolleko]

Good idea, but i don't like the fact though that we are making diffing internals public/internal.
Will think about it.