Remove dependency on argument-parser (swiftlang#149)

Removes swift-markdown's dependency on swift-argument-parser by
refactoring the markdown-tool target into its own package.

Markdown-tool is largely intended as a tool for developers working on
swift-markdown so there isn't a need to vend it to clients. Removing
argument-parser as a dependency allows for more clients to depend
on swift-markdown since it removes any potential conflicts with
different argument-parser versions.

Author: ethan-kusters

Package.swift

@@ -29,12 +29,6 @@ let package = Package(
                 .product(name: "cmark-gfm", package: cmarkPackageName),
                 .product(name: "cmark-gfm-extensions", package: cmarkPackageName),
             ]),
-        .executableTarget(
-            name: "markdown-tool",
-            dependencies: [
-                "Markdown",
-                .product(name: "ArgumentParser", package: "swift-argument-parser")
-            ]),
         .testTarget(
             name: "MarkdownTests",
             dependencies: ["Markdown"],
@@ -50,7 +44,6 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
     // Building standalone, so fetch all dependencies remotely.
     package.dependencies += [
         .package(url: "https://github.com/apple/swift-cmark.git", branch: "gfm"),
-        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.2.2"),
     ]
 
     // SwiftPM command plugins are only supported by Swift version 5.6 and later.
@@ -63,6 +56,5 @@ if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
     // Building in the Swift.org CI system, so rely on local versions of dependencies.
     package.dependencies += [
         .package(path: "../cmark"),
-        .package(path: "../swift-argument-parser"),
     ]
 }

Tools/Package.swift

@@ -0,0 +1,33 @@
+// swift-tools-version:5.5
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2023 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import PackageDescription
+
+let package = Package(
+    name: "Tools",
+    products: [
+        .executable(name: "markdown-tool", targets: ["markdown-tool"]),
+    ],
+    dependencies: [
+        .package(name: "swift-markdown", path: "../."),
+        .package(url: "https://github.com/apple/swift-argument-parser", from: "1.2.2"),
+    ],
+    targets: [
+        .executableTarget(
+            name: "markdown-tool",
+            dependencies: [
+                .product(name: "Markdown", package: "swift-markdown"),
+                .product(name: "ArgumentParser", package: "swift-argument-parser")
+            ],
+            path: "markdown-tool"
+        ),
+    ]
+)