add pkg updating support

[eyberg]

we have support for updating nanos and ops but not individual packages - when updating i typically blow away the old one which while works should be explicit, the other thing is that we check hashes on initial download and ops will complain if they aren't right

end-user (not pkg creator) prob should have an update pkg cmd, while there are arguments for keeping multiple builds of same pkg name/same version we don't have that support server-side yet, the end result is that the user might not have something that is up to date if someone was working on a pkg