Use local git package

Author: kimdv

CodeGeneration/Package.swift

@@ -10,10 +10,6 @@ let package = Package(
   products: [
     .executable(name: "generate-swiftsyntax", targets: ["generate-swiftsyntax"])
   ],
-  dependencies: [
-    .package(url: "https://github.com/apple/swift-syntax.git", revision: "94120e0cf9bf541e54fbc8512934daea23a9ad0c"),
-    .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMinor(from: "1.2.2")),
-  ],
   targets: [
     .executableTarget(
       name: "generate-swiftsyntax",
@@ -41,3 +37,16 @@ let package = Package(
     ),
   ]
 )
+
+if ProcessInfo.processInfo.environment["SWIFTCI_USE_LOCAL_DEPS"] == nil {
+    // Building standalone.
+    package.dependencies += [
+        .package(url: "https://github.com/apple/swift-syntax.git", revision: "94120e0cf9bf541e54fbc8512934daea23a9ad0c"),
+        .package(url: "https://github.com/apple/swift-argument-parser.git", .upToNextMinor(from: "1.2.2")),
+    ]
+} else {
+    package.dependencies += [
+        .package(path: ".."),
+        .package(path: "../../swift-argument-parser"),
+    ]
+}