Hoping to boost water supplies during future droughts, Silicon Valley’s largest water provider is working on a plan to build a new $800 million dam and reservoir in the remote hills of eastern Santa Clara County, just off Pacheco Pass. The idea, still in the early stages, could result in the construction of one of the largest reservoirs in the Bay Area — a lake that would be twice the size of Crystal Springs Reservoir along Interstate 280 in San Mateo County — and the first new reservoir built in Santa Clara County since 1957, when Uvas Reservoir near Morgan Hill opened.