Operation Packification, step 2 of tons.
Eventually the buildlet client binary will use this stuff now.
Change-Id: I4cf5f3e6beb9e56bdc795ed513ce6daaf61425e3
Reviewed-on: https://go-review.googlesource.com/2921
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
The new package will be used by the buildlet command line tool in the
future.
Also use the metadata package in the coordinator for the master build key,
since it was already depending on that package. Might as well use it.
Change-Id: I6ad7593787ebba364090951ccbaecb4aa38e1dd6
Reviewed-on: https://go-review.googlesource.com/2920
Reviewed-by: Andrew Gerrand <adg@golang.org>