Document an exhaustive list of packages shipped in or used by a given version of the .NET SDK

Users that maintain package feed caches need to know what to populate on release day, so we should have a machine-readable list of all packages the SDK might grab, either as Package Downloads or Implicit Package References.

Categories of packages that should be included (not exhaustive):
* Runtimes
* AppHosts
* SingleFileHosts
* ILLink and related tasks

Once this machine-readable list is generated it should be a release artifact and should be used to power:
* automated linkages for the release.jsons
* human-readable markdown documentation in the release notes and detailed developer/administrator documentation.