“The Lord of the Flies ” is the kind of show you praise by emphasizing how hard it is to watch. Adapted from William Golding’s classic 1954 novel by Jack Thorne (co-writer of “Adolescence”), directed by Marc Munden (“The Sympathizer”) and originally aired by the BBC before coming to Netflix in the U.S., the four-episode series doesn’t make any major changes to Golding’s potent allegory for the thin line separating civilization from savagery. The story of British schoolboys marooned on a remote tropical island without adult supervision isn’t modernized — it retains its World War II backdrop — or gender-swapped, like Showtime’s “Flies”-inflected “Yellowjackets.” It also doesn’t have to be. Simply watching these boys, played by a uniformly terrific cast of child actors, succumb to their worst instincts is harrowing enough to make you long to look away — even if you’d be missing some gripping drama.…