It’s often said that there are only seven basic plots in literature. When it comes to biographies of rock stars who died young, by contrast, there’s usually just the one: somebody mysteriously talented emerges from an unlikely background to achieve stardom, before being destroyed by drink, drugs and fame. Yet, as the film Amy proved last year, it’s a plot still capable of packing a real emotional punch — and, as Cowboy Song proves now, the life of Phil Lynott from Thin Lizzy embodies it more vividly than most.