feat: support multiple ExportsPlugin declarations

Editor/API/Attributes/ExportsPlugin.cs

@@ -17,7 +17,7 @@ namespace nadena.dev.ndmf
     /// }
     /// </code>
     /// </summary>
-    [AttributeUsage(AttributeTargets.Assembly)]
+    [AttributeUsage(AttributeTargets.Assembly, AllowMultiple = true)]
     public sealed class ExportsPlugin : Attribute
     {
         public Type PluginType { get; }

UnitTests~/PluginResolverTests/SupportsMultipleDeclarations.cs

@@ -0,0 +1,43 @@
+using System.Collections.Immutable;
+using System.Linq;
+using nadena.dev.ndmf;
+using NUnit.Framework;
+using UnitTests.ExportsPluginTest;
+
+[assembly: ExportsPlugin(typeof(PluginA))]
+[assembly: ExportsPlugin(typeof(PluginB))]
+
+namespace UnitTests.ExportsPluginTest
+{
+    internal class PluginA : Plugin<PluginA>
+    {
+        protected override void Configure()
+        {
+            InPhase(BuildPhase.Transforming).Run("test", _ctx => { });
+        }
+    }
+
+    internal class PluginB : Plugin<PluginB>
+    {
+        protected override void Configure()
+        {
+            InPhase(BuildPhase.Transforming).Run("test", _ctx => { });
+        }
+    }
+
+    public class SupportsMultipleDeclarations
+    {
+        [Test]
+        public void TestSupportsMultipleDeclarations()
+        {
+            var resolver = new PluginResolver();
+            var plugins =
+                resolver.Passes.SelectMany(kv => kv.Item2) // passes per phase
+                    .Select(pass => pass.Plugin.GetType())
+                    .ToImmutableHashSet();
+
+            Assert.IsTrue(plugins.Contains(typeof(PluginA)));
+            Assert.IsTrue(plugins.Contains(typeof(PluginB)));
+        }
+    }
+}