[Xamarin.Android.Build.Tasks] remove Distinct() from <ResolveLibraryP…

…rojectImports/>

The `<ResolveLibraryProjectImports/>` task has some logic that needs
to look at the item metadata of each `@(Reference)`:

* `%(AndroidSkipResourceExtraction)` to skip unzipping anything
* `%(AndroidSkipResourceProcessing)` to skip `<ConvertResourcesCases/>`

To make matters weird, we had some LINQ:

    foreach (var assemblyPath in Assemblies
            .Select (a => a.ItemSpec)
            .Distinct ()) {

And we would basically lose the metadata information...

To handle this problem, we had two `HashSet`'s we used to lookup the
values:

    assembliesToSkipCaseFixup = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
    assembliestoSkipExtraction = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
    bool metaDataValue;
    foreach (var asm in Assemblies) {
        if (bool.TryParse (asm.GetMetadata (AndroidSkipResourceProcessing), out metaDataValue) && metaDataValue)
            assembliesToSkipCaseFixup.Add (asm.ItemSpec);
        if (bool.TryParse (asm.GetMetadata (GetAdditionalResourcesFromAssemblies.AndroidSkipResourceExtraction), out metaDataValue) && metaDataValue)
            assembliestoSkipExtraction.Add (asm.ItemSpec);
    }

I added a test to see what happens when we use multiple `@(Reference)`
such as:

* `<PackageReference/>`
* `packages.config` and a regular `<Reference/>`
* `<Reference/>` with a full path

It turns out there are MSBuild targets that disambiguate duplicate
assembly references:

https://github.com/NuGet/NuGet.BuildTasks/blob/640c8e13a9b7ab6e86264a296638fbf3cc016ad1/src/Microsoft.NuGet.Build.Tasks/Microsoft.NuGet.targets#L186-L207

The `ResolveNuGetPackageAssets` does a `Remove` against duplicate
`@(Reference)`, even if there are no NuGet packages present in the
project.

It seems safe for us to just drop the `Distinct()` and the
`HashSet`'s. The code is a bit simpler after doing this.

I'm also seeing a small ~25ms performance improvement with this change.

src/Xamarin.Android.Build.Tasks/Tasks/ResolveLibraryProjectImports.cs

@@ -67,7 +67,6 @@ public class ResolveLibraryProjectImports : Task
 		};
 
 		AssemblyIdentityMap assemblyMap = new AssemblyIdentityMap();
-		HashSet<string> assembliesToSkipCaseFixup, assembliestoSkipExtraction;
 
 		public ResolveLibraryProjectImports ()
 		{
@@ -83,16 +82,6 @@ public override bool Execute ()
 			var resolvedEnvironmentFiles      = new List<ITaskItem> ();
 
 			assemblyMap.Load (AssemblyIdentityMapFile);
-			assembliesToSkipCaseFixup = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
-			assembliestoSkipExtraction = new HashSet<string> (StringComparer.OrdinalIgnoreCase);
-			bool metaDataValue;
-			foreach (var asm in Assemblies) {
-				if (bool.TryParse (asm.GetMetadata (AndroidSkipResourceProcessing), out metaDataValue) && metaDataValue)
-					assembliesToSkipCaseFixup.Add (asm.ItemSpec);
-				if (bool.TryParse (asm.GetMetadata (GetAdditionalResourcesFromAssemblies.AndroidSkipResourceExtraction), out metaDataValue) && metaDataValue)
-					assembliestoSkipExtraction.Add (asm.ItemSpec);
-			}
-
 			using (var resolver = new DirectoryAssemblyResolver (this.CreateTaskLogger (), loadDebugSymbols: false)) {
 				try {
 					Extract (resolver, jars, resolvedResourceDirectories, resolvedAssetDirectories, resolvedEnvironmentFiles);
@@ -170,10 +159,9 @@ void Extract (
 			foreach (var assembly in Assemblies)
 				res.Load (assembly.ItemSpec);
 
-			// FIXME: reorder references by import priority (not sure how to do that yet)
-			foreach (var assemblyPath in Assemblies
-					.Select (a => a.ItemSpec)
-					.Distinct ()) {
+			bool skip;
+			foreach (var assemblyItem in Assemblies) {
+				var assemblyPath = assemblyItem.ItemSpec;
 				var fileName = Path.GetFileName (assemblyPath);
 				if (MonoAndroidHelper.IsFrameworkAssembly (fileName) &&
 						!MonoAndroidHelper.FrameworkEmbeddedJarLookupTargets.Contains (fileName) &&
@@ -185,7 +173,7 @@ void Extract (
 					Log.LogDebugMessage ($"Skipping non-existent dependency '{assemblyPath}' during a design-time build.");
 					continue;
 				}
-				if (assembliestoSkipExtraction.Contains (assemblyPath)) {
+				if (bool.TryParse (assemblyItem.GetMetadata (GetAdditionalResourcesFromAssemblies.AndroidSkipResourceExtraction), out skip) && skip) {
 					Log.LogDebugMessage ("Skipping resource extraction for '{0}' .", assemblyPath);
 					continue;
 				}
@@ -216,7 +204,7 @@ void Extract (
 						var taskItem = new TaskItem (Path.GetFullPath (resDir), new Dictionary<string, string> {
 							{ OriginalFile, assemblyPath },
 						});
-						if (assembliesToSkipCaseFixup.Contains (assemblyPath))
+						if (bool.TryParse (assemblyItem.GetMetadata (AndroidSkipResourceProcessing), out skip) && skip)
 							taskItem.SetMetadata (AndroidSkipResourceProcessing, "True");
 						resolvedResourceDirectories.Add (taskItem);
 					}
@@ -320,7 +308,7 @@ void Extract (
 							var taskItem = new TaskItem (Path.GetFullPath (resDir), new Dictionary<string, string> {
 								{ OriginalFile, assemblyPath }
 							});
-							if (assembliesToSkipCaseFixup.Contains (assemblyPath))
+							if (bool.TryParse (assemblyItem.GetMetadata (AndroidSkipResourceProcessing), out skip) && skip)
 								taskItem.SetMetadata (AndroidSkipResourceProcessing, "True");
 							resolvedResourceDirectories.Add (taskItem);
 						}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -134,6 +134,29 @@ public void BuildBasicApplicationAppCompat ([Values (true, false)] bool usePacka
 			}
 		}
 
+		[Test]
+		public void DuplicateReferences ()
+		{
+			var proj = new XamarinAndroidApplicationProject ();
+			proj.MainActivity = proj.DefaultMainActivity.Replace ("public class MainActivity : Activity", "public class MainActivity : Android.Support.V7.App.AppCompatActivity");
+			var package = KnownPackages.SupportV7AppCompat_27_0_2_1;
+			var fullPath = Path.GetFullPath (Path.Combine (Root, "temp", "packages", $"{package.Id}.{package.Version}", "lib", package.TargetFramework, $"{package.Id}.dll"));
+			proj.PackageReferences.Add (package);
+			proj.Packages.Add (package);
+			proj.References.Add (new BuildItem.Reference (package.Id) {
+				MetadataValues = "HintPath=" + fullPath,
+			});
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
+				Assert.IsTrue (b.Build (proj), "first build should have succeeded.");
+
+				// Remove NuGet packages, but leave References
+				proj.PackageReferences.Clear ();
+				proj.Packages.Clear ();
+
+				Assert.IsTrue (b.Build (proj), "second build should have succeeded.");
+			}
+		}
+
 		[Test]
 		public void BuildXamarinFormsMapsApplication ()
 		{