Commands Refactor Part 2

We split #196 in two parts. this is the second. (#262 is the first). This builds on all the changes to commands/.
This should be `importer.BuildDagFromReader`, otherwise it will not work as required! Don't change logic!! _only_ port. if something needs to be improved (in this case it isn't), that should be a separate PR afterwards.

Also, this was changed recently... are you sure you're porting commands as they stand today?