With these changes, goinstall is now able to use branches
maintained with Bazaar located in Launchpad.

Project aliases such as /project and /project/series are
supported in addition to specific user or team branches
such as /~user/project/branch.  Temporary branches under
the +junk special project are also supported.

As a curious side effect, since Launchpad is able to import
code from other locations, they can be indirectly
accessible too if desired.

R=rsc
CC=golang-dev
https://golang.org/cl/1699050