Adding support for database literal references for Dacpac and SqlProj references (#1858)

* fixing up some nullable spots

* Adding database literal support

* Adding tests and support for same database refs

* Fixing test

* merge laggard

* Split database reference tests up

* PR feedback

* Normalizing strings for cross-plat test passing

* Updating Projects nuget package

* Fixing up test

test/Microsoft.SqlTools.ServiceLayer.UnitTests/SqlProjects/SqlProjectTests.cs

@@ -0,0 +1,37 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System;
+using Microsoft.SqlTools.ServiceLayer.SqlProjects.Contracts;
+using NUnit.Framework;
+
+namespace Microsoft.SqlTools.ServiceLayer.UnitTests.SqlProjects
+{
+    public class SqlProjectTests
+    {
+        [Test]
+        public void DatabaseReferenceValidationTest()
+        {
+            // Verify that Validate() throws when both DatabaseLiteral and DatabaseVariable are set 
+            AddUserDatabaseReferenceParams reference = new AddDacpacReferenceParams() // any concrete class will do
+            {
+                DatabaseLiteral = "DatabaseName",
+                DatabaseVariable = "$(DatabaseVariable)"
+            };
+
+            Assert.Throws<ArgumentException>(() => reference.Validate(), $"Validate() for a reference with both {nameof(reference.DatabaseLiteral)} and {nameof(reference.DatabaseVariable)} should have failed");
+
+            // Verify that Validate() passes any other time
+            reference = new AddDacpacReferenceParams() { DatabaseLiteral = "DatabaseName" };
+            reference.Validate();
+
+            reference = new AddDacpacReferenceParams() { DatabaseVariable = "$(DatabaseVariable)" };
+            reference.Validate();
+
+            reference = new AddDacpacReferenceParams();
+            reference.Validate();
+        }
+    }
+}