Did some amateur research. Hope this helps.
Shieding / rolling seldom seems to work against committed projectile spammers, since they'll usually just keep firing even after your shield is dropped and during the roll's recovery animation; try jumping and approaching from the air or using Monkey Flip to cover distance and punish with a grab. This is especially useful if they to to shield.
If they roll, punish. Learn their animations and hit them when they're getting back onto their feet.
Remember that you can't flinch from Fox's blaster. Falco's and Pit's projectiles will make you flinch, but they're also much slower and more difficult to fire repeatedly. Whereas Fox can keep firing indefinitely, Falco and Pit have to cool down before firing again. Spot-dodging between shots can actually work wonders when approaching.
As the above poster said, if you're feeling brave, just work up the guts to dash right through the shots. You may take some damage, but if you're not standing there and taking it, they won't be able to damage you for too long. Do your best to avoid being a target. If you do what you can to fight back against committed projectile spammers, they won't be able to exploit you and you can adapt to defeat 'em.
As I say, I play the likes of Pit, Palutena, R.O.B. and Ludwig, so if you need help sparring with projectile-based fighters, message me and we'll work it out!