"Commands" and "Env" don't need to unpack full plan

[jhclark]

Currently, the "env" and "commands" modes, calculate and unpack the full plan, before outputting the commands. This is unnecessary and slow.

The fix is to generalize Plans.getPlannedVertices() to take a taskName and realiazationNames as strings instead of a plan. This fix will then also allow us to easily specify a target task to run on the command line.