Implement RestorePackages in MSBuild.

This adds the DotNetTool custom task, with `dotnet restore` implemented.

build_projects/dotnet-cli-build/DotNetTool.cs

@@ -0,0 +1,85 @@
+using Microsoft.Build.Framework;
+using Microsoft.Build.Utilities;
+
+namespace Microsoft.DotNet.Cli.Build
+{
+    public abstract class DotNetTool : ToolTask
+    {
+        private const string ExeName = "dotnet.exe";
+
+        public DotNetTool()
+        {
+        }
+
+        protected abstract string Command { get; }
+
+        protected abstract string Args { get; }
+
+        public string WorkingDirectory { get; set; }
+
+        protected override string ToolName
+        {
+            get { return ExeName; }
+        }
+
+        protected override MessageImportance StandardOutputLoggingImportance
+        {
+            get { return MessageImportance.High; } // or else the output doesn't get logged by default
+        }
+
+        protected override string GenerateFullPathToTool()
+        {
+            string path = ToolPath;
+
+            // if ToolPath was not provided by the MSBuild script 
+            if (string.IsNullOrEmpty(path))
+            {
+                Log.LogError($"Could not find the Path to {ExeName}");
+
+                return string.Empty;
+            }
+
+            return path;
+        }
+
+        protected override string GetWorkingDirectory()
+        {
+            return WorkingDirectory ?? base.GetWorkingDirectory();
+        }
+
+        protected override string GenerateCommandLineCommands()
+        {
+            return $"{Command} {Args}";
+        }
+
+        protected override void LogToolCommand(string message)
+        {
+            base.LogToolCommand($"{GetWorkingDirectory()}> {message}");
+        }
+    }
+
+    public class DotNetRestore : DotNetTool
+    {
+        protected override string Command
+        {
+            get { return "restore"; }
+        }
+
+        protected override string Args
+        {
+            get { return $"{GetVerbosity()}"; }
+        }
+
+        public string Verbosity { get; set; }
+
+        private string GetVerbosity()
+        {
+            if (!string.IsNullOrEmpty(Verbosity))
+            {
+                return $"--verbosity {Verbosity}";
+            }
+
+            return null;
+        }
+    }
+}