[swift-syntax] Update for underscored product names

(cherry picked from commit 3f8d13d)

Author: rintaro

lib/ASTGen/Package.swift

@@ -52,7 +52,7 @@ let package = Package(
     .target(
       name: "swiftASTGen",
       dependencies: [
-        .product(name: "SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
+        .product(name: "_SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
         .product(name: "SwiftDiagnostics", package: "swift-syntax"),
         .product(name: "SwiftOperators", package: "swift-syntax"),
         .product(name: "SwiftParser", package: "swift-syntax"),

tools/swift-plugin-server/Package.swift

@@ -18,15 +18,15 @@ let package = Package(
     .executableTarget(
       name: "swift-plugin-server",
       dependencies: [
-        .product(name: "SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
-        .product(name: "SwiftLibraryPluginProvider", package: "swift-syntax"),
+        .product(name: "_SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
+        .product(name: "_SwiftLibraryPluginProvider", package: "swift-syntax"),
       ]
     ),
     .target(
       name: "SwiftInProcPluginServer",
       dependencies: [
-        .product(name: "SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
-        .product(name: "SwiftLibraryPluginProvider", package: "swift-syntax"),
+        .product(name: "_SwiftCompilerPluginMessageHandling", package: "swift-syntax"),
+        .product(name: "_SwiftLibraryPluginProvider", package: "swift-syntax"),
       ]
     ),
   ],