SEATTLE — Aberdeen’s efforts to keep visitors out of a longstanding homeless encampment have drawn a federal lawsuit. People have lived at Riverfront Camp — formerly known as Hobo Beach — along the Chehalis River for more than a century. The site used to be private property, but the city bought the land in August with plans to clear it out, saying it wasn’t fit for human habitation. Aberdeen said it would allow residents to remain through the winter if they registered, and more than 100 did.