Improve the granularity and flexibility of support declarations

In preparation for the implementation of ArchiveReorganisation/Components, we need a richer interface for declaration of "supported". There are two current methods by which support is indicated: firstly each package may have a Supported: status indicating whether it will be provided with 18, 36, or 60 months of security support by the Canonical security team, and secondly many of the package management tools (including comments in sources.list) use separation by component to declare support by Canonical.

A richer interface should allow arbitrary (authorised) parties to declare support for arbitrary terms for arbitrary sets of packages, with package collections based on (potentially overlapping) packagesets or other types of list, rather than on component.