Add template support for creating a command plugin package

This is fairly minimal at the moment, and doesn't try to be a tutorial on how to write command plugins.  Needs a unit test.

rdar://107288352

Author: neonichu

Sources/Commands/PackageTools/Init.swift

@@ -31,6 +31,7 @@ extension SwiftPackageTool {
                 tool              - A package with an executable that uses
                                     Swift Argument Parser. Use this template if you
                                     plan to have a rich set of command-line arguments.
+                command-plugin    - A package that vends a command plugin.
                 macro             - A package that vends a macro.
                 empty             - An empty package with a Package.swift manifest.
                 """))

Sources/Workspace/InitPackage.swift

@@ -2,7 +2,7 @@
 //
 // This source file is part of the Swift open source project
 //
-// Copyright (c) 2014-2020 Apple Inc. and the Swift project authors
+// Copyright (c) 2014-2023 Apple Inc. and the Swift project authors
 // Licensed under Apache License v2.0 with Runtime Library Exception
 //
 // See http://swift.org/LICENSE.txt for license information
@@ -44,7 +44,7 @@ public final class InitPackage {
         case library = "library"
         case executable = "executable"
         case tool = "tool"
-        case `extension` = "extension"
+        case commandPlugin = "command-plugin"
         case macro = "macro"
 
         public var description: String {
@@ -115,6 +115,7 @@ public final class InitPackage {
         // none of it exists, and then act.
         try writeManifestFile()
         try writeGitIgnore()
+        try writePlugins()
         try writeSources()
         try writeTests()
     }
@@ -270,6 +271,14 @@ public final class InitPackage {
                                 ]),
                         ]
                     """
+                } else if packageType == .commandPlugin {
+                    param += """
+                            .plugin(
+                                 name: "\(typeName)",
+                                 capability: .command(intent: .custom(verb: "\(typeName)", description: "prints hello world"))
+                             ),
+                        ]
+                    """
                 } else if packageType == .macro {
                     param += """
                             // Macro implementation, only built for the host and never part of a client program.
@@ -351,8 +360,44 @@ public final class InitPackage {
         }
     }
 
+    private func writePlugins() throws {
+        switch packageType {
+        case .commandPlugin:
+            let plugins = destinationPath.appending(component: "Plugins")
+            guard self.fileSystem.exists(plugins) == false else {
+                return
+            }
+            progressReporter?("Creating \(plugins.relative(to: destinationPath))/")
+            try makeDirectories(plugins)
+
+            let moduleDir = plugins.appending(component: "\(pkgname)")
+            try makeDirectories(moduleDir)
+
+            let sourceFileName = "plugin.swift"
+            let sourceFile = try AbsolutePath(validating: sourceFileName, relativeTo: moduleDir)
+
+            let content = """
+                import PackagePlugin
+
+                @main
+                struct \(typeName): CommandPlugin {
+                    func performCommand(context: PluginContext, arguments: [String]) async throws {
+                        print("Hello, World!")
+                    }
+                }
+            """
+
+            try writePackageFile(sourceFile) { stream in
+                stream.write(content)
+            }
+
+        case .empty, .library, .executable, .tool, .macro:
+            break
+        }
+    }
+
     private func writeSources() throws {
-        if packageType == .empty {
+        if packageType == .empty || packageType == .commandPlugin {
             return
         }
 
@@ -428,7 +473,7 @@ public final class InitPackage {
             public macro stringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "\(moduleName)Macros", type: "StringifyMacro")
             """
 
-        case .empty, .`extension`:
+        case .empty, .commandPlugin:
             throw InternalError("invalid packageType \(packageType)")
         }
 
@@ -444,7 +489,7 @@ public final class InitPackage {
 
     private func writeTests() throws {
         switch packageType {
-        case .empty, .executable, .tool, .`extension`: return
+        case .empty, .executable, .tool, .commandPlugin: return
             default: break
         }
         let tests = destinationPath.appending("Tests")
@@ -590,7 +635,7 @@ public final class InitPackage {
 
         let testClassFile = try AbsolutePath(validating: "\(moduleName)Tests.swift", relativeTo: testModule)
         switch packageType {
-        case .empty, .`extension`, .executable, .tool: break
+        case .empty, .commandPlugin, .executable, .tool: break
         case .library:
             try writeLibraryTestsFile(testClassFile)
         case .macro: