[xamlc] fix duplicate Mono.Cecil search paths (#12030)

I was building the Control Gallery and saw:

    XamlCTask
        Compiling Xaml, assembly: obj\Debug\netstandard2.0\Xamarin.Forms.Controls.dll
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        Adding searchpath C:\Users\jopepper\.nuget\packages\newtonsoft.json\12.0.3\lib\netstandard2.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\nunit\3.12.0\lib\netstandard2.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\xam.plugin.deviceinfo\3.0.2\lib\netstandard1.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\xam.plugin.deviceinfo\3.0.2\lib\netstandard1.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        ...

122 lines in total.

In f330652 there was an attempt to fix this in the past with a
`.Distinct()` call, but `Path.GetDirectoryName()` wasn't called first.

After reordering things, the `<XamlCTask/>` now prints 9 search paths
for the Control Gallery:

    XamlCTask
        Compiling Xaml, assembly: obj\Debug\netstandard2.0\Xamarin.Forms.Controls.dll
        Adding searchpath C:\Users\jopepper\.nuget\packages\netstandard.library\2.0.3\build\netstandard2.0\ref
        Adding searchpath C:\Users\jopepper\.nuget\packages\newtonsoft.json\12.0.3\lib\netstandard2.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\nunit\3.12.0\lib\netstandard2.0
        Adding searchpath C:\Users\jopepper\.nuget\packages\xam.plugin.deviceinfo\3.0.2\lib\netstandard1.0
        Adding searchpath C:\src\Xamarin.Forms\Xamarin.Forms.Core\bin\Debug\netstandard2.0
        Adding searchpath C:\src\Xamarin.Forms\Xamarin.Forms.CustomAttributes\bin\Debug\netstandard2.0
        Adding searchpath C:\src\Xamarin.Forms\Xamarin.Forms.Maps\bin\Debug\netstandard2.0
        Adding searchpath C:\src\Xamarin.Forms\Xamarin.Forms.Platform\bin\Debug\netstandard2.0
        Adding searchpath C:\src\Xamarin.Forms\Xamarin.Forms.Xaml\bin\Debug\netstandard2.0

Other changes to `<XamlCTask/>`:

* Made `ReferencePath` a `string[]` so that MSBuild can split the item
  groups for us. This way we don't have to call `string.Split()` and
  weird (and valid!) separators work such as space characters or %3b.

* Removed `DependencyPaths`, as I could not find any usage of it.

~~ Results ~~

I could see small performance difference fixing this.

On Windows + .NET framework:

    Before:
    2140 ms  XamlCTask                                  1 calls
    After:
    2074 ms  XamlCTask                                  1 calls

On macOS + Mono:

    Before:
    7580 ms  XamlCTask                                  1 calls
    After:
    7480 ms  XamlCTask                                  1 calls

My PC is newer/faster than my Mac, but I think this probably saves
between 65ms-100ms for `<XamlCTask/>` in the Control Gallery sample.

Author: jonathanpeppers

Xamarin.Forms.Build.Tasks/DebugXamlCTask.cs

@@ -17,16 +17,9 @@ public override bool Execute(out IList<Exception> thrownExceptions)
 			LoggingHelper.LogMessage(Normal, $"{new string(' ', 0)}Preparing debug code for xamlc, assembly: {Assembly}");
 
 			var resolver = new DefaultAssemblyResolver();
-			if (!string.IsNullOrEmpty(DependencyPaths)) {
-				foreach (var dep in DependencyPaths.Split(';')) {
-					LoggingHelper.LogMessage(Low, $"{new string(' ', 2)}Adding searchpath {dep}");
-					resolver.AddSearchDirectory(dep);
-				}
-			}
-			if (!string.IsNullOrEmpty(ReferencePath)) {
-				var paths = ReferencePath.Replace("//", "/").Split(';');
-				foreach (var p in paths) {
-					var searchpath = IOPath.GetDirectoryName(p);
+			if (ReferencePath != null) {
+				var paths = ReferencePath.Select(p => IOPath.GetDirectoryName(p.Replace("//", "/"))).Distinct();
+				foreach (var searchpath in paths) {
 					LoggingHelper.LogMessage(Low, $"{new string(' ', 2)}Adding searchpath {searchpath}");
 					resolver.AddSearchDirectory(searchpath);
 				}

Xamarin.Forms.Build.Tasks/XamlCTask.cs

@@ -53,17 +53,9 @@ public override bool Execute(out IList<Exception> thrownExceptions)
 			using (var fallbackResolver = DefaultAssemblyResolver == null ? new XamlCAssemblyResolver() : null) {
 				var resolver = DefaultAssemblyResolver ?? fallbackResolver;
 				if (resolver is XamlCAssemblyResolver xamlCResolver) {
-					if (!string.IsNullOrEmpty(DependencyPaths)) {
-						foreach (var dep in DependencyPaths.Split(';').Distinct()) {
-							LoggingHelper.LogMessage(Low, $"{new string(' ', 2)}Adding searchpath {dep}");
-							xamlCResolver.AddSearchDirectory(dep);
-						}
-					}
-
-					if (!string.IsNullOrEmpty(ReferencePath)) {
-						var paths = ReferencePath.Replace("//", "/").Split(';').Distinct();
-						foreach (var p in paths) {
-							var searchpath = IOPath.GetDirectoryName(p);
+					if (ReferencePath != null) {
+						var paths = ReferencePath.Select (p => IOPath.GetDirectoryName(p.Replace("//", "/"))).Distinct();
+						foreach (var searchpath in paths) {
 							LoggingHelper.LogMessage(Low, $"{new string(' ', 2)}Adding searchpath {searchpath}");
 							xamlCResolver.AddSearchDirectory(searchpath);
 						}

Xamarin.Forms.Build.Tasks/XamlTask.cs

@@ -22,8 +22,7 @@ public abstract class XamlTask : MarshalByRefObject, ITask
 	{
 		[Required]
 		public string Assembly { get; set; }
-		public string DependencyPaths { get; set; }
-		public string ReferencePath { get; set; }
+		public string[] ReferencePath { get; set; }
 		[Obsolete("this is no longer used")]
 		[EditorBrowsable(EditorBrowsableState.Never)]
 		public int Verbosity { get; set; }

Xamarin.Forms.Xaml.UnitTests/MockCompiler.cs

@@ -26,7 +26,7 @@ public static void Compile(Type type, out MethodDefinition methdoDefinition)
 
 			var xamlc = new XamlCTask {
 				Assembly = assembly,
-				ReferencePath = string.Join(";", refs),
+				ReferencePath = refs.ToArray(),
 				KeepXamlResources = true,
 				OptimizeIL = true,
 				DebugSymbols = false,