- Don't prompt if only one package to build/drop [bug karen]

  In build/drop commands don't show the plan or ask for confirmation if
  only one package to be built. Maybe even better if the set of packages
  is the same as on the command line.

  With build the tricky aspect is if the package is being upgraded or
  downgraded. Perhaps only omit plan/prompt if it will only contain
  'build' actions?