[6.0] Add swift package add-target-dependency command to edit the manifest (swiftlang#7628)

**Explanation**: Building on SE-0301, add a new command `swift package
add-target-dependency` to edit the package manifest and add a new
dependency to a given target.
**Original** PR:
swiftlang#7594
**Risk**: Low. New, very simple code path to edit the package manifest
in a narrow way.
**Reviewed by**: @MaxDesiatov

Author: DougGregor

Package.swift

@@ -751,6 +751,7 @@ if ProcessInfo.processInfo.environment["SWIFTCI_DISABLE_SDK_DEPENDENT_TESTS"] ==
                 "Build",
                 "Commands",
                 "PackageModel",
+                "PackageModelSyntax",
                 "PackageRegistryCommand",
                 "SourceControl",
                 "SPMTestSupport",

Sources/Commands/CMakeLists.txt

@@ -10,6 +10,7 @@ add_library(Commands
   PackageCommands/AddDependency.swift
   PackageCommands/AddProduct.swift 
   PackageCommands/AddTarget.swift
+  PackageCommands/AddTargetDependency.swift
   PackageCommands/APIDiff.swift
   PackageCommands/ArchiveSource.swift
   PackageCommands/CompletionCommand.swift

Sources/Commands/PackageCommands/AddTargetDependency.swift

@@ -0,0 +1,89 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2014-2024 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
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import ArgumentParser
+import Basics
+import CoreCommands
+import PackageModel
+import PackageModelSyntax
+import SwiftParser
+import SwiftSyntax
+import TSCBasic
+import TSCUtility
+import Workspace
+
+extension SwiftPackageCommand {
+    struct AddTargetDependency: SwiftCommand {
+        package static let configuration = CommandConfiguration(
+            abstract: "Add a new target dependency to the manifest")
+
+        @OptionGroup(visibility: .hidden)
+        var globalOptions: GlobalOptions
+
+        @Argument(help: "The name of the new dependency")
+        var dependencyName: String
+
+        @Argument(help: "The name of the target to update")
+        var targetName: String
+
+        @Option(help: "The package in which the dependency resides")
+        var package: String?
+
+        func run(_ swiftCommandState: SwiftCommandState) throws {
+            let workspace = try swiftCommandState.getActiveWorkspace()
+
+            guard let packagePath = try swiftCommandState.getWorkspaceRoot().packages.first else {
+                throw StringError("unknown package")
+            }
+
+            // Load the manifest file
+            let fileSystem = workspace.fileSystem
+            let manifestPath = packagePath.appending("Package.swift")
+            let manifestContents: ByteString
+            do {
+                manifestContents = try fileSystem.readFileContents(manifestPath)
+            } catch {
+                throw StringError("cannot find package manifest in \(manifestPath)")
+            }
+
+            // Parse the manifest.
+            let manifestSyntax = manifestContents.withData { data in
+                data.withUnsafeBytes { buffer in
+                    buffer.withMemoryRebound(to: UInt8.self) { buffer in
+                        Parser.parse(source: buffer)
+                    }
+                }
+            }
+
+            let dependency: TargetDescription.Dependency
+            if let package {
+                dependency = .product(name: dependencyName, package: package)
+            } else {
+                dependency = .target(name: dependencyName, condition: nil)
+            }
+
+            let editResult = try PackageModelSyntax.AddTargetDependency.addTargetDependency(
+                dependency,
+                targetName: targetName,
+                to: manifestSyntax
+            )
+
+            try editResult.applyEdits(
+                to: fileSystem,
+                manifest: manifestSyntax,
+                manifestPath: manifestPath,
+                verbose: !globalOptions.logging.quiet
+            )
+        }
+    }
+}
+

Sources/Commands/PackageCommands/SwiftPackageCommand.swift

@@ -36,6 +36,7 @@ public struct SwiftPackageCommand: AsyncParsableCommand {
             AddDependency.self,
             AddProduct.self,
             AddTarget.self,
+            AddTargetDependency.self,
             Clean.self,
             PurgeCache.self,
             Reset.self,

Tests/CommandsTests/PackageCommandTests.swift

@@ -849,6 +849,38 @@ final class PackageCommandTests: CommandsTestCase {
         }
     }
 
+    func testPackageAddTargetDependency() throws {
+        try testWithTemporaryDirectory { tmpPath in
+            let fs = localFileSystem
+            let path = tmpPath.appending("PackageB")
+            try fs.createDirectory(path)
+
+            try fs.writeFileContents(path.appending("Package.swift"), string:
+                """
+                // swift-tools-version: 5.9
+                import PackageDescription
+                let package = Package(
+                    name: "client",
+                    targets: [ .target(name: "library") ]
+                )
+                """
+            )
+            try localFileSystem.writeFileContents(path.appending(components: "Sources", "library", "library.swift"), string:
+                """
+                public func Foo() { }
+                """
+            )
+
+            _ = try execute(["add-target-dependency", "--package", "other-package", "other-product", "library"], packagePath: path)
+
+            let manifest = path.appending("Package.swift")
+            XCTAssertFileExists(manifest)
+            let contents: String = try fs.readFileContents(manifest)
+
+            XCTAssertMatch(contents, .contains(#".product(name: "other-product", package: "other-package"#))
+        }
+    }
+
     func testPackageAddProduct() throws {
         try testWithTemporaryDirectory { tmpPath in
             let fs = localFileSystem