build-from-branch into the primary archive

[Martin Pool]

I'd like to do a new feature in Launchpad to help Ubuntu, which is to
allow packages to be built directly from source package branches.
This would address one of the messy parts of source package branches
at the moment, which is the need to upload the package, mark it
uploaded, and also push the branch.  This will build on things that
seems to be quite popular and successful with ppa building from
recipes, and help us simplify some redundantc

Most of what we need to do this already exists, so I'm hoping that
we'll be able to get it up quite quickly.

I would like to start on this soon by just offering it as an adjunct
to regular dput uploads, for a limited set of packages. For packages
in that set, when someone commits to the branch, a part of Soyuz will
automatically assemble a source package and queue it for building.
This will let people try it out while still having the option to keep
using regular dput uploads.

We have a Launchpad Enhancement Proposal (LEP) about this at
<https://dev.launchpad.net/LEP/BuildFromBranchIntoPrimary>.  I'd
appreciate hearing of

 * any problems you can spot in this
 * any missing constraints or likely snags we ought to consider
 * anyone or any packages who'd like to be first to try it

Thanks!
Martin