Problem 1: The type attribute in the costmap_plugins.xml should refer to the namespaced version of the implementation class. Ergo costmap_plugins::StaticCostmap should be static_costmap_plugin::StaticCostmapPlugin.
Problem 2: The same applies to the first parameter of PLUGINLIB_DECLARE_CLASS
PLUGINLIB_DECLARE_CLASS(static_costmap_plugin, static_costmap, static_costmap_plugin::StaticCostmapPlugin, costmap_2d::CostmapPlugin)