Runtime/Platform-specific package build artifact import

This seems to be a

### Steps to reproduce

1. Following the instructions [in the CoreFX repo](https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/package-projects.md#platform-specific-library) to create a platform-specific library with an associated metapackage. (e.g. Microsoft.DotNet.FooPackage and runtime.win-x64.Microsoft.DotNet.FooPackage).
2. In the platform-specific package add a .targets file with the name of the package under a `build` folder.
3. Push the created packages to a package feed (local or remote).
4. Create any simple .NET Core app template and add a reference to the created package. If not already added, also add the package feed as a package source.
5. Specify a property in the .csproj file of the app with its target runtime. ( e.g. `win-x64`)
6. Run `dotnet restore`

### Expected behavior
The build artifact in the runtime-specific package is added to the app's nuget.g.targets/props files. This seems to be the case for MSBuild outside of .NET Core.

### Actual behavior
The build artifact in the runtime-specific package is not added to the app's nuget.g.targets/props files.

### Environment data
`msbuild /version` output: 15.5.180.51428

OS info: Microsoft Windows 10 Enterprise Version 10.0.16299 Build 16299

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc): dotnet cli 2.1.4-preview-007237