Fixed #35508: Add an --ignore-deps flag to squashmigrations (WIP)

[pygeek]

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Provide a concise overview of the issue or rationale behind the proposed changes.

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[shaib]

I see that this is just initial, so no detailed review, I just raised points which I suspect will save you some time in further work

[shaib]

This should probably be put under else:

[pygeek]

I was working through the logic, at that point, prior to changing the existing code. I just committed a more complete solution. Perhaps, you could clarify something else for me, however.

1. How are test files created by migration commands cleaned up? I only see references for rewinding the state, but not deletion of created migration files.
2. Should I exclude multi-app ModelOperations altogether, or mutate them to exclude fields that reference other apps?
3. What would you recommend doing in the case of a SwappableTuple? Is there a predictable way to check if it's referencing an external app? Let me know if the logic I currently have makes sense.

I'm still writing tests. Also, let me know if I missed anything.

PS: I may decide to check the dependencies for multi-app dependencies, before combing through all of the operations to find references to other apps.