Builds lists should filter out builds for superseded sources

[Julian Edwards]

Builds lists (IHasBuildRecords implentations) should display only builds for current source packages, i.e. filter out builds for source packages that are superseded. This is different from superseded builds, which are builds that will never happen because they were superseded before they were done. We also need to filter out builds which were done, and which may or may not have resulted in binaries which are still in the distro, but which are builds for superseded sources. (NB: We could also ensure builds are superseded more efficiently).