No documented definition of package sets

[Kristian Glass]

I couldn't find a useful definition of what a package set actually _is_ (beyond "a set of packages")

* https://launchpad.net/+apidoc/1.0.html#packageset doesn't provide/link to a definition
* https://help.launchpad.net/API?action=fullsearch&titlesearch=0&value=%22package+set%22&context=180 only returns https://help.launchpad.net/Packaging/BuildScores?highlight=%28package+set%29 which provides a limited definition of package sets but (reasonably) only in the context of build scores

Potentially useful things might include:

* Usage, implications, relation to distros, etc.
* Its "type" - it's _not_ just a set of packages, it can contain other package sets too (source: cjwatson)

Possible (partial) information sources include:

* https://dev.launchpad.net/Soyuz/Specs/PackagesetsAndDistroseries
* https://dev.launchpad.net/SoyuzPackageSetAcl
* https://help.launchpad.net/Packaging/BuildScores