Add Vcs-*: fields to packages

[geofft]

There are  a couple of cool fields such as Vcs-Svn: that we can take advantage of, in part to make dasource a thin wrapper around  debcheckout.

However, because we use different authenticated and unauthenticated SVN paths, this is only really useful for development if we introduce something like a Vcs-Svn-Auth: field that lets you (with some additional option to debcheckout) get a source checkout that you can commit from.

[geofft]

While we expect this to not be hard / take that long, it doesn't need to be milestoned for any particular release and isn't blocking anything release-specific.