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:

go-graphsync/benchmarks/benchmark_test.go

Lines 273 to 276 in a8d8802

	dagService := merkledag.NewDAGService(blockservice.New(bs, offline.Exchange(bs)))
	// import to UnixFS
	bufferedDS := ipldformat.NewBufferedDAG(ctx, dagService)

@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.

[hannahhoward]

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