Automatically reference shared frameworks from dependencies

[dsplaisted]

If a dependency of a project (either another project in the solution, or a NuGet package) depends on an optional shared framework, then the project with the dependency should also depend on that shared framework.

This depends on NuGet writing these dependencies to the assets file: NuGet/Home#7342. Once that is done we will raise them into framework references.

We will also need to download all targeting packs for the targeted framework in case we might need to depend on them via a transitive dependency (which we won't know about until after restore has happened).

NOTE: We would also like to have a property to disable transitive framework references (which would then disable the automatic download of packages you might not need).