Make splitting submodules private #2626

Job	Run time
Demos, examples, and docs	4m 31s
Evaluation-based tests	3m 54s
Minimal dependency tests (all options)	24m 29s
Minimal dependency tests	16m 47s
Rust test coverage	16m 23s
Vanilla Python 3.12.5 on windows-latest	19m 30s
Vanilla Python 3.13 on ubuntu-24.04-arm	14m 41s
Vanilla Python 3.12.5 on macos-latest	12m 2s
Vanilla Python 3.13 on macos-latest	22m 46s
Vanilla Python 3.12.5 on ubuntu-24.04-arm	16m 32s
Vanilla Python 3.14t on ubuntu-latest	11m 26s
Vanilla Python 3.14 on ubuntu-latest	11m 44s
Vanilla Python 3.14 on ubuntu-24.04-arm	10m 42s
Vanilla Python 3.13 on ubuntu-latest	14m 15s
Vanilla Python 3.14t on ubuntu-24.04-arm	11m 26s
Vanilla Python 3.14t on windows-latest	15m 42s
Vanilla Python 3.14 on windows-latest	14m 34s
Vanilla Python 3.13 on windows-latest	17m 50s
Vanilla Python 3.14t on macos-latest	14m 54s
Vanilla Python 3.14 on macos-latest	16m 55s
Vanilla Python 3.12.5 on ubuntu-latest	15m 0s
Test suite results	23s
	5h 6m 26s

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