Use the GenerateRestoreGraphFile MSBuild target to deterine the
package spec information instead of determining this from the project
model. This allows multi-targeting projects and conditional
PackageReferences to be supported.

Fixes VSTS #583482 - PackageReference does not respect condition
Fixes VSTS #572320 - Support conditional NuGet PackageReferences in
multi-targeting projects
Fixes VSTS #572329 - Show per-framework dependencies when multi-targeting
Fixes VSTS #563083 - Using Paket with new .fsproj format