Move `Checkpointer` implementations to `_src`. #12295

Job	Run time
build-checkpoint (Python 3.10, jax=newest)	1m 13s
build-export (Python 3.10, jax=newest)	1m 42s
build-checkpoint (Python 3.11, jax=newest)	1m 10s
build-export (Python 3.10, jax=0.4.30)	1m 49s
build-checkpoint (Python 3.10, jax=0.4.34)	1m 10s
	7m 4s

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