Nuget.Client repo with csproj.

CSProjs.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>netcoreapp2.0</TargetFramework>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="NuGet.Client" Version="4.2.0" />
    <PackageReference Include="NuGet.Configuration" Version="4.4.0" />
  </ItemGroup>
</Project>

Program.cs

using System;
using System.Linq;
using NuGet;
using NuGet.Configuration;
using NuGet.Repositories;

namespace CSProjs
{
    class Program
    {
        static void Main(string[] args)
        {
            var repo = new NuGetv3LocalRepository(SettingsUtility.GetGlobalPackagesFolder(Settings.LoadDefaultSettings(null)));
            var packages = repo.FindPackagesById("NuGet.Client");
            var pkg = packages.Last();
            var nuspec = pkg.Nuspec;
        }
    }
}