replace uses of go-merkledag and go-ipld-format to create unixfs files with go-unixfsnode

[mvdan]

For example, in the code below, we use go-merkledag and go-ipld-format to create a UnixFS file for a benchmark:

https://github.com/ipfs/go-graphsync/blob/a8d880238c2679e6191d67d26dd859896c32237f/benchmarks/benchmark_test.go#L273-L276

@willscott says we can use data/builder.BuildUnixFSFile for this purpose:

https://github.com/ipfs/go-unixfsnode/blob/b9b6e9dc571e3fd0a3e4ee53987f07e0ff8008da/data/builder/file.go#L30

Hopefully this would mean that the root module no longer needs to pull those two dependencies, which are relatively large and getting phased out with time.

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[hannahhoward]

ooh if go-unixfs node has gotten this far we should for SURE do this.