Support any namespace in NuspecReader (#278)

The NuspecReader was hardcoded to always assume the nuspec v6 schema.
However, the NuGet client will use the _oldest_ schema it can when
creating a package.

Update the reader so it uses whatever the provided XML schema is.

Author: MattKotsenas

src/Microsoft.Build.Utilities.ProjectCreation.UnitTests/NuspecReaderTests.cs

@@ -34,5 +34,29 @@ public void LicenseExpressionIsNullForFileLicenses()
             nuspec.LicenseVersion.ShouldBeNull();
             nuspec.LicenseUrl.ShouldBeNull();
         }
+
+        // List of namespaces from https://github.com/NuGet/NuGet.Client/blob/6917e6c883d45f45672e20d4b1c45758dad2fa84/src/NuGet.Core/NuGet.Packaging/PackageCreation/Authoring/ManifestSchemaUtility.cs#L23-L50
+        [Theory]
+        [InlineData("http://schemas.microsoft.com/packaging/2010/07/nuspec.xsd")]
+        [InlineData("http://schemas.microsoft.com/packaging/2011/08/nuspec.xsd")]
+        [InlineData("http://schemas.microsoft.com/packaging/2011/10/nuspec.xsd")]
+        [InlineData("http://schemas.microsoft.com/packaging/2012/06/nuspec.xsd")]
+        [InlineData("http://schemas.microsoft.com/packaging/2013/01/nuspec.xsd")]
+        [InlineData("http://schemas.microsoft.com/packaging/2013/05/nuspec.xsd")]
+        public void ReaderCanParseKnownXmlNamespaces(string xmlns)
+        {
+            string contents =
+$@"<package xmlns=""{xmlns}"">
+  <metadata>
+    <id>PackageL</id>
+    <version>16.4.60</version>
+  </metadata>
+</package>";
+
+            NuspecReader nuspec = new NuspecReader(contents);
+
+            nuspec.Id.ShouldBe("PackageL");
+            nuspec.Version.ShouldBe("16.4.60");
+        }
     }
 }

src/Microsoft.Build.Utilities.ProjectCreation/NuspecReader.cs

@@ -5,6 +5,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 using System.Xml;
 using System.Xml.Linq;
 using System.Xml.XPath;
@@ -13,17 +14,15 @@ namespace Microsoft.Build.Utilities.ProjectCreation
 {
     internal class NuspecReader
     {
-        private static readonly XNamespace NuspecNamespace = "http://schemas.microsoft.com/packaging/2013/05/nuspec.xsd";
-
         private readonly XDocument _document;
-
         private readonly XmlNamespaceManager _namespaceManager = new XmlNamespaceManager(new NameTable());
 
         public NuspecReader(string contents)
         {
             _document = XDocument.Parse(contents);
 
-            _namespaceManager.AddNamespace("ns", NuspecNamespace.NamespaceName);
+            string ns = GetNamespace(_document);
+            _namespaceManager.AddNamespace("ns", ns);
         }
 
         public NuspecReader(FileInfo fileInfo)
@@ -125,6 +124,21 @@ public IEnumerable<string> PackageTypes
 
         public string? Version => GetElement("version");
 
+        private static string GetNamespace(XDocument document)
+        {
+            XPathNavigator navigator = document.CreateNavigator();
+            navigator.MoveToFollowing(XPathNodeType.Element);
+
+            IDictionary<string, string> namespaces = navigator.GetNamespacesInScope(XmlNamespaceScope.ExcludeXml);
+
+            if (namespaces.Count != 1)
+            {
+                throw new XmlException($"Unabled to determine nuspec XML namespace. Namespaces in scope: '{string.Join(",", new Dictionary<string, string>(namespaces))}'.");
+            }
+
+            return namespaces.Values.Single();
+        }
+
         private IEnumerable<PackageContentFileEntry> GetContentFiles()
         {
             foreach (XElement file in _document.XPathSelectElements("//ns:package/ns:metadata/ns:contentFiles/ns:files", _namespaceManager))