Create a swift bundler command line plugin.

Package.swift

@@ -6,7 +6,8 @@ let package = Package(
   name: "swift-bundler",
   platforms: [.macOS(.v11)],
   products: [
-    .executable(name: "swift-bundler", targets: ["swift-bundler"])
+    .executable(name: "swift-bundler", targets: ["swift-bundler"]),
+    .plugin(name: "SwiftBundlerCommandPlugin", targets: ["SwiftBundlerCommandPlugin"]),
   ],
   dependencies: [
     .package(url: "https://github.com/stackotter/swift-arg-parser", from: "1.2.3"),
@@ -59,5 +60,22 @@ let package = Package(
       path: "Documentation",
       exclude: ["preview_docs.sh"]
     ),
+
+    .plugin(
+      name: "SwiftBundlerCommandPlugin",
+      capability: .command(
+        intent: .custom(
+          verb: "bundler",
+          description: "Run a package as an app."
+        ),
+        permissions: [
+          .writeToPackageDirectory(
+            reason: "Creating an app bundle requires writing to the package directory.")
+        ]
+      ),
+      dependencies: [
+        .target(name: "swift-bundler")
+      ]
+    ),
   ]
 )

Plugins/SwiftBundlerCommandPlugin/SwiftBundlerCommandPlugin.swift

@@ -0,0 +1,45 @@
+import Foundation
+import PackagePlugin
+
+@main
+struct SwiftBundlerCommandPlugin: CommandPlugin {
+  /// This entry point is called when operating on a Swift package.
+  func performCommand(context: PluginContext, arguments: [String]) throws {
+    let bundler = try context.tool(named: "swift-bundler")
+
+    try run(command: bundler.path, with: arguments)
+  }
+}
+
+#if canImport(XcodeProjectPlugin)
+  import XcodeProjectPlugin
+
+  extension SwiftBundlerCommandPlugin: XcodeCommandPlugin {
+    /// This entry point is called when operating on an Xcode project.
+    func performCommand(context: XcodePluginContext, arguments: [String]) throws {
+      let bundler = try context.tool(named: "swift-bundler")
+
+      try run(command: bundler.path, with: arguments)
+    }
+  }
+#endif
+
+extension SwiftBundlerCommandPlugin {
+  /// Run a command with the given arguments.
+  func run(command: Path, with arguments: [String]) throws {
+    let exec = URL(fileURLWithPath: command.string)
+
+    let process = try Process.run(exec, arguments: arguments)
+    process.waitUntilExit()
+
+    // Check whether the subprocess invocation was successful.
+    if process.terminationReason == .exit,
+      process.terminationStatus == 0
+    {
+      print("SwiftBundlerCommandPlugin succesfully completed.")
+    } else {
+      let problem = "\(process.terminationReason):\(process.terminationStatus)"
+      Diagnostics.error("SwiftBundlerCommandPlugin failed: \(problem)")
+    }
+  }
+}