Iirc, some snakes have some heat sensing cells arranged in pits to give them a directional heat sense. If you did that with visible light instead of IR those pits would be what halfway to an eye would look like
Like light sensitive cells these would need a similar 1 - 4 +++, so the point would really be the same.
A cell sensing light can use the same type of nerves etc as other sensory cells, so there is no need to explain how a cell sensing light + a nerve + a central nervous system evolves in one step.
If you wonder how a nerve could evolve for example, that is a different question. If you have two cells and they can exchange chemical signals, which is useful in itself, you have the start of a nerve.