Good idea... probably stashing in the .julia directory rather than METADATA. I'll try it out. @StefanKarpinski do you think that will work? 

Also, a single resolve might install multiple packages. Currently if one of them fails (currently possible due to a bad url, for eg), everything already done in that call stays as is, and the command returns, leaving everything else un-done. Presumably we want the whole resolve call to be a single transaction?