fix error message and rename variables to clarify what they are

[goddenrich]

previously we were

t := graph.WaitForTarget(l)
if t == nil {
			return fmt.Errorf("... (provided by %s), ...", ..., t)

which was not helpful given it will be nil. What I believe was intended was to put the target resolve in line one of the function

t := graph.WaitForTarget(*dep.declared)

which was being masked within the for loop. I have made this change and renamed the variables to be clearer