MICHIGAN—Arson, black market hash, a decade-old law nobody remembers, and free joints for folks impacted by the shutdown of Spirit Airlines. It’s another week in Michigan weed. Spark up and catch up: MINE FIELD: A decade-old state law signed by former Gov. Rick Snyder could be triggered by federal marijuana rescheduling, potentially creating a new licensing structure that would allow weed to be sold at pharmacies like CVS or Walgreens, Crain’s D…