message: expose datasource components
diff: export * as PropTypes from "./types";
  +export Viewer from "./Viewer";
  +
  export Entity from "./Entity";
  +
  +export DataSource from "./DataSource";
  +export CustomDataSource from "./CustomDataSource";
  +export CzmlDataSource from "./CzmlDataSource";
  +export GeoJsonDataSource from "./GeoJsonDataSource";
  +export KmlDataSource from "./KmlDataSource";
  +
  +export Primitive from "./Primitive";
  export PointPrimitive from "./PointPrimitive";
  export PointPrimitiveCollection from "./PointPrimitiveCollection";
  -export Primitive from "./Primitive";
  -export Viewer from "./Viewer";
  