Project build system support for ref assemblies (PCL bait-and-switch)

From @fubar-coder on 24 May, 2016

Copied from: [dotnet/cli#3211](https://github.com/dotnet/cli/issues/3211)
## Steps to reproduce

You want to create a bait-and-switch NuGet package by specifying a build for the `ref` folder instead of the `lib` folder.
## Expected behavior

A way to specify in the csproj/xproj/project.json that the build output is packaged to the `ref`, instead of the `lib` folder.
## Actual behavior

No way to include it except by using `packInclude` (or similar means).