Support macro targets in plugin context

Sources/PackagePlugin/PackageModel.swift

@@ -235,6 +235,9 @@ public enum ModuleKind {
     case snippet
     /// A module that contains unit tests.
     case test
+    /// A module that contains code for a macro.
+    @available(_PackageDescription, introduced: 5.9)
+    case macro // FIXME: This should really come from `CompilerPluginSupport` somehow, but we lack the infrastructure to allow that currently.
 }
 
 /// Represents a target consisting of a source code module compiled using Swift.

Sources/PackagePlugin/PluginContextDeserializer.swift

@@ -261,6 +261,8 @@ fileprivate extension ModuleKind {
             self = .snippet
         case .test:
             self = .test
+        case .macro:
+            self = .macro
         }
     }
 }

Sources/PackagePlugin/PluginMessages.swift

@@ -173,6 +173,7 @@ enum HostToPluginMessage: Codable {
                         case executable
                         case snippet
                         case test
+                        case macro
                     }
 
                     enum BinaryArtifactKind: Codable {

Sources/SPMBuildCore/PluginContextSerializer.swift

@@ -278,7 +278,9 @@ fileprivate extension WireInput.Target.TargetInfo.SourceModuleKind {
             self = .snippet
         case .test:
             self = .test
-        case .binary, .plugin, .systemModule, .macro:
+        case .macro:
+            self = .macro
+        case .binary, .plugin, .systemModule:
             throw StringError("unexpected target kind \(kind) for source module")
         }
     }