ResolveNuGetPackageAssets.ReferencedPackages returns the wrong result when dependencies are listed under framework

Sample project:

```
{
"dependencies": {
"System.Runtime": "4.0.0",
"System.Dynamic.Runtime": "4.0.0",
"System.Linq.Expressions": "4.0.0"
},
"frameworks": {
"dotnet5.1": {
"dependencies": {
"System.IO": "4.0.0"
}
}
}
}
```

When building this I would expect the build task to return System.IO in the list of ReferencedPackages. It does not.

Potentially hacky fix: https://github.com/dotnet/buildtools/pull/368

Moved from: https://github.com/NuGet/Home/issues/1843

cc @ericstj @jasonmalinowski