[BE] Move submodule check out from setup.py (#8315)

* [BE] Move submodule check out from setup.py

Summary: Per comment in #8203: https://github.com/pytorch/executorch/pull/8203#discussion_r1943763809

Moving the logic of checking submodule and update out from setup.py and
into install_executorch.py.

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* lint

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* Move submodule handling to before install requirements

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