For decades, just about the only way for someone born in Dawan to make money was to follow the winding dirt road down from the mountain village and move to one of China’s big cities. That is what Wang Liangcui did in the early 1990s, when she was 20. She landed in Shanghai, where she worked in factories, drove a taxi and hawked pancakes. All over China, people just like her moved to the city from similar hardscrabble hamlets, supplying the inexpensive labor that got the nation’s economy sizzling. Now,…