Split out `VisibleStore` and `ReferrersStore` from `Store` #6886

Job	Run time
Check Cachix and Docker secrets present for installer tests	1s
tests (ubuntu-latest)	16m 59s
tests (macos-latest)	22m 55s
docker_push_image	0s
installer	0s
installer_test	0s
	39m 55s

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