Microsoft.Extensions.Testing.Abstractions is the only one I couldn't move to netstandard1.3 that should be. But this library should be removed with the new `dotnet test` strategy.
1. Rebase on new LibraryExporter
2. Update dependency name to "fx/<name>" for reference assembly
3. Update framework friendly name
4. Fix dependency message regression
5. Update tests
- Ported nuget package building code over from dnu. Moved that code to use NuGet v3 primitives.
- Simplified the package builder API
- Left out resources and schema detection for now
- This folder should remain self contained as the code will be copied into NuGet v3.
- Missing features include symbols packages
