quilt: Let users specify spec path in quilt run
quilt: Let users specify spec path in quilt run

In this patch, users can now reference specs relative to their
current directory or relative to `QUILT_PATH`, when running a
spec.

LuiseV/quilt,tejasmanohar/quilt,stevenlin598/quilt,stevenlin598/quilt,ejj/quilt-dev,quilt/quilt,LuiseV/quilt,tejasmanohar/quilt,kelda/kelda,mmussomele/quilt,NetSys/quilt,kayousterhout/quilt,kelda/kelda,secant/quilt,stevenlin598/quilt,ZhongtianWang/quilt,PenguinToast/quilt,stevenlin598/quilt,mmussomele/quilt,secant/quilt,kklin/quilt,stevenlin598/quilt,akshayknarayan/quilt,secant/quilt,ejj/quilt-dev,kklin/kelda,kklin/kelda,kklin/quilt,kelda/kelda,secant/quilt,PenguinToast/quilt,PenguinToast/quilt,kklin/quilt,quilt/quilt,NetSys/quilt,kklin/quilt,mmussomele/quilt,PenguinToast/quilt,ZhongtianWang/quilt,akshayknarayan/quilt,ZhongtianWang/quilt,PenguinToast/quilt,LuiseV/quilt,akshayknarayan/quilt,kklin/kelda,kayousterhout/quilt,quilt/quilt,akshayknarayan/quilt,ejj/quilt-dev,NetSys/quilt,ZhongtianWang/quilt,mmussomele/quilt,kayousterhout/quilt,secant/quilt,NetSys/quilt,LuiseV/quilt,kelda/kelda,ejj/quilt-dev,mmussomele/quilt,LuiseV/quilt,kklin/kelda,NetSys/quilt,tejasmanohar/quilt,kayousterhout/quilt,stevenlin598/quilt,quilt/quilt,tejasmanohar/quilt,mmussomele/quilt