SCOTTSDALE — Trevor McDonald’s locker is five down from Landen Roupp’s in the Giants’ spring training clubhouse, and it would make a lot of sense if Roupp’s path is the one McDonald hopes to follow. Roupp was a starter in the minors but was so good two springs ago that the staff had no choice but to put him in the Opening Day bullpen. After making 19 relief appearances and four starts that season, he won a starting job the next spring. Roupp is now locked into the rotation.There’s not currently a starting spot available for McDonald, but like Roupp two years ago, he looks like one of the best 13 arms in camp. And he doesn’t need anybody to give him a recent history lesson.“Me and him are best friends and ride to and from the field every day and talk about that stuff,” McDonald said. “Whether it’s the bullpen, rotation, whatever, my main goal is just wherever they need me, be ready to be there and compete when I get the opportunity.”McDonald has looked sharp all spring, and he struck out a pair in a quick inning Monday afternoon against the A’s. Like many of the other young arms in camp, he also...