Fix error when installing EF.SharpDevelop NuGet package.

Enumerating the EnvDTE.Project.Object.References in a different app domain
was trying to load the PackageManagement assembly and failing. Changed
EnvDTE.References so it only exposes types in the SharpDevelop.EnvDTE assembly
and switch to using IEnumerable on the EnvDTE.References class.

src/AddIns/Misc/PackageManagement/Project/Src/EnvDTE/References.cs

@@ -10,7 +10,7 @@
 
 namespace ICSharpCode.PackageManagement.EnvDTE
 {
-	public class References : MarshalByRefObject, IEnumerable<Reference>, global::EnvDTE.References
+	public class References : MarshalByRefObject, IEnumerable, global::EnvDTE.References
 	{
 		MSBuildBasedProject msbuildProject;
 		IPackageManagementProjectService projectService;
@@ -45,24 +45,18 @@ void SaveProject()
 			project.Save();
 		}
 
-		public IEnumerator<Reference> GetEnumerator()
+		public IEnumerator GetEnumerator()
 		{
-			List<Reference> references = GetReferences().ToList();
-			return references.GetEnumerator();
+			return GetReferences().ToList().GetEnumerator();
 		}
 
-		IEnumerable<Reference> GetReferences()
+		IEnumerable<global::EnvDTE.Reference> GetReferences()
 		{
 			foreach (ReferenceProjectItem referenceProjectItem in project.GetReferences()) {
 				yield return new Reference3(project, referenceProjectItem);
 			}
 		}
 
-		IEnumerator IEnumerable.GetEnumerator()
-		{
-			return GetEnumerator();
-		}
-
 		public global::EnvDTE.Reference Item(string identity)
 		{
 			return Find(identity);

src/AddIns/Misc/PackageManagement/Test/Src/EnvDTE/ReferencesTests.cs

@@ -30,18 +30,10 @@ void CreateReferences()
 			references = (References)projectObject.References;
 		}
 
-		void ReferenceCollectionAssertAreEqual(string[] expectedReferences, List<Reference> referenceList)
-		{
-			var actualReferences = new List<string>();
-			referenceList.ForEach(r => actualReferences.Add(r.Name));
-
-			CollectionAssert.AreEqual(expectedReferences, actualReferences);
-		}
-
 		void ReferenceCollectionAssertAreEqual(string[] expectedReferences, IEnumerable referenceList)
 		{
 			var actualReferences = new List<string>();
-			foreach (Reference reference in referenceList) {
+			foreach (global::EnvDTE.Reference reference in referenceList) {
 				actualReferences.Add(reference.Name);
 			}
 
@@ -116,15 +108,14 @@ public void GetEnumerator_ProjectHasTwoReferences_TwoReferencesReturned()
 			msbuildProject.AddReference("System.Data");
 			msbuildProject.AddReference("System.Xml");
 
-			var referenceList = new List<Reference>();
-			referenceList.AddRange(references);
+			var enumerable = references as IEnumerable;
 
 			var expectedReferences = new string[] {
 				"System.Data",
 				"System.Xml"
 			};
 
-			ReferenceCollectionAssertAreEqual(expectedReferences, referenceList);
+			ReferenceCollectionAssertAreEqual(expectedReferences, enumerable);
 		}
 
 		[Test]
@@ -134,14 +125,13 @@ public void GetEnumerator_ProjectHasFileItemBeforeReferenceItem_OneReferenceRetu
 			msbuildProject.AddFile(@"src\Test.cs");
 			msbuildProject.AddReference("System.Xml");
 
-			var referenceList = new List<Reference>();
-			referenceList.AddRange(references);
+			var enumerable = references as IEnumerable;
 
 			var expectedReferences = new string[] {
 				"System.Xml"
 			};
 
-			ReferenceCollectionAssertAreEqual(expectedReferences, referenceList);
+			ReferenceCollectionAssertAreEqual(expectedReferences, enumerable);
 		}
 
 		[Test]