Consider a single location for version history

[jdreed]

In many cases (machtype, gconf2-config, and now dconf), we need to know the "history" of Debathena. This can be as simple as all the previous versions we've supported, in order, or it can be more complicated ("What are all the versions in which we've cared about dconf?"). Rather than each package needing to provide this, and then us forgetting to update it (gconf2), I propose a single location where this information is maintained, that packages can query.

[andersk]

These exist (package distro-info):
distro-info --all
ubuntu-distro-info --all
debian-distro-info --all