There’s been a blizzard of commentary about the breakdown of U.S.-Canadian trade talks. Friend of TPM Paul Krugman has a good rundown of just how dependent the U.S. is on key Canadian exports — oil, lumber, hydropower electricity in addition to many other commodities. Of course, the U.S. is a net exporter of oil. But that’s in aggregate. As Paul explains, the details aren’t that simple. The upper Midwest gets most of its oil from Canada while a …