Remove dependency to merkledag in go-car/v2

[masih]

merkledag dependency in go-car/v2 is used purely in testing. It carries with it a number of heavy dependencies. Remove and use alternatives in testing.

[MichaelMure]

It would be neat (not just for this project) to have an equivalent of https://github.com/ipfs/go-ipfs-blocksutil to generate DAGs and nodes for testing. That would resolve that issue as far as I can tell.

[rvagg]

I think most of the use of merkledag is just NewRawNode() which could be fairly easily replaced by a new utility function that does the same. There's a few ProtoNode uses but they could also be replaced by direct use of go-codec-dagpb (which merkledag uses anyway).

The biggest sticking point is probably in internal/carv1/car.go, which isn't just for testing, where we do a merkledag.Walk. This could be replaced with a go-ipld-prime traversal but it's not as simple as a one-line replacement.

But I do agree that we should remove it, the more of these old libraries we can detach from here the better.