Use dune data_only_dirs stanza instead of sed to ignore benchmarks

[punchagan]

Adding and removing directories from this list is easier and much more stable than writing sed rules. It also moves all the benchmark filtering to a single place and makes it easy to see which benchmarks are being ignored.

We use dune's data_only_dirs stanza to tell dune to ignore dune files in some directories. This stanza rule only allows specifying immediate sub-directories. So, we create a benchmarks/dune file and add the ignore rules to it.

[shakthimaan]

Good enhancement!