As controversial as the Paul McCartney Archive Collection’s approach to Flowers in the Dirt has been, the official release of demos from McCartney’s fruitful writing sessions with Elvis Costello has made for some excitement around Second Disc HQ. Some of those fateful “McCartney/McManus” demos are now slated for an exclusive Record Store Day piece, it was announced yesterday. Three unheard cassette demos, part of the much-maligned download-only portion of the deluxe box set (which will also feature previously released B-sides and remixes, instead of this content on a physical disc), have been…