The packages is called a "heatsink slug", and trade names of "Barracuda" (*), by Lumileds Corp, or "Dragon" by Osram Opto.
in the chapter on LED packaging of the somewhat-retired LED-related website Light-Emitting-Diodes-dot-org - (note different domain).
See the bottom-left image: Chapter 11: Packaging - Fig 11.5.
*Barracuda was, at least initially, actually referring to the type of LED die itself, not the package. See ledmuseum.candlepower.us and Philips LED Update - page 8