Add support for NuGet restore / assets file reading to MSBuild

Support for NuGet PackageReference restore and build is currently spread across NuGet.targets and the .NET SDK. This prevents non .NET projects from supporting NuGet PackageReference.

We would like to make NuGet PackageReference available to all MSBuild projects. That includes the following:

- Targets required to send data to NuGet (nomination)
- Mostly works, apart from setting assets file path
- Targets required to make restore work from the command line - these already seem to be imported from common targets
- Targets required to make dependency node work
- Read data from assets file and return dependencies (now only top-level dependencies)
- Targets required to make build work
- Reading assets file, create appropriate items (ie Content) from assets file