C#: Move external api declarations to the library pack.

csharp/ql/src/Telemetry/TestLibrary.qll

@@ -1,16 +1,4 @@
-private import csharp
+// Use `semmle.code.csharp.telemetry.TestLibrary` instead.
+deprecated module;
 
-pragma[nomagic]
-private predicate isTestNamespace(Namespace ns) {
-  ns.getFullName()
-      .matches([
-          "NUnit.Framework%", "Xunit%", "Microsoft.VisualStudio.TestTools.UnitTesting%", "Moq%"
-        ])
-}
-
-/**
- * A test library.
- */
-class TestLibrary extends RefType {
-  TestLibrary() { isTestNamespace(this.getNamespace()) }
-}
+import semmle.code.csharp.telemetry.TestLibrary