currently the fs-specific objects (file format, dagreader, etc.) are spread out across the importer and the merkledag packages.

But the filesystem-specific stuff does not belong in regular merkledag. merkledag is lower level, it only provides the dag link structure. the fs is one of many possible datastructures on top.

We should probably have a unixfs package that implements the dir/file abstractions, with indirect blocks, and with its reader/writers within an unixfs.io subpackage.