These functions are specific to handling packages stored in the
cache, not other files. They are also always used with the same
`cache/packages.` prefix to the path.
Gbp-Dch: Short
Closes: #952916
* Add function to output unique list of foreign architectures from an
expanded package list.
* If foreign architectures are detected, add unique architectures
to dpkg and update apt.
* This requires users to explicitly list at least _one_ package of a
foreign architecture in their package list (e.g. foo:arch) for any other
foreign arch dependencies to be handled appropriately.
Previously, we needed to keep the lb_* prefix as the scripts
could also be executed standalone. Since the lb wrapper is
mandatory since squeeze, we can drop this now.