Acquire/Consume assets from runtime pack for self-contained publish

[dsplaisted]

For .NET Core 3.0, we plan to use runtime packs to deliver runtime assets for self-contained publish.

We expect to use "download only" packages to acquire the appropriate runtime pack. This means that the runtime assets will no longer be in the assets file. So we will need an alternate way of consuming these assets. They will also need to flow through conflict resolution.

This issue also covers acquiring the correct runtime pack (by adding a "download only" package reference for restore).