debian package imports should check if Vcs-Bzr points to a UDD branch

For several packages I maintain in Debian that previously were not maintained in VCSes, I've found it useful to branch from the UDD branches to use as the authoritative branches of my packages. This is useful to me as a Debian maintainer, but Ubuntu can't benefit from any shared history going forward because UDD reimports based on uploaded tarballs and never looks at my divergent bzr branch.

It would not be impossible for UDD to look at these branches, however; they are all documented in Vcs-Bzr fields in my source packages. E.g.:

  Vcs-Bzr: nosmart+http://bzr.debian.org/bzr/users/vorlon/libdbd-sybase-perl/trunk/

So I think the UDD package importer should look at these. Suggested pseudocode:

  look at the debian/control file in the new Debian source package version to be imported
  if vcs-bzr field is present:
    examine the named branch
    if the named branch is reachable, has the necessary shared history to be pullable, and has proper UDD-style tags for this package/upstream version:
      pull from the branch
      compare contents with the downloaded source package, and if there's a mismatch, give precedence to the source package (like we already do for the Ubuntu archive)
      next package
  fall back to importing from the source package