Migration: Do not add RIDs for Library projects (#5279)

* WIP

* Do not add runtime identifiers for libraries

* Reusing an existing test project

* Fix up tests

TestAssets/TestProjects/PJLibWithMultipleFrameworks/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Library
+{
+    public class TestLib
+    {
+        public static void Test()
+        {
+            Console.WriteLine("Hello World!");
+        }
+    }
+}

TestAssets/TestProjects/PJLibWithMultipleFrameworks/project.json

@@ -0,0 +1,16 @@
+{
+  "version": "1.0.0-*",
+  "frameworks": {
+    "netstandard1.6": {
+      "dependencies": {
+        "NETStandard.Library": "1.6"
+      }
+    },
+    "netstandard1.3": {
+      "dependencies": {
+        "NETStandard.Library": "1.3"
+      }
+    },
+    "net451": {}
+  }
+}