We’ve lived with the Kentucky genius-freak Will Oldham long enough for us to take him for granted, and for his best work to disappear. Johnny Cash covered the title track from I See A Darkness, the bleakest of the American bummer-folk masterpieces that darkened the late 90s, an era when our country had become richer and more powerful than it had been or maybe ever would be again. Naturally, the album is no longer on Spotify. The Letting Go has been similarly let go of — you must turn to YouTube, or, God forbid, physical media to experience Oldham’s 2006 creative peak, a great poet’s cry across the outer and inner wilderness, powered through spare vocal harmonies and a restless churn of guitar.  Oldham, whose latest album was released under his Bonnie Prince Billy alias this past June, is now one of the last survivors of a golden age of existential dark Americana. David Berman, Elliott Smith, and Low’s Mimi Parker are all dead. Mark Kozelek of the Red House Painters was hounded out of …