Remove @_implementationOnly annotations

These annotations produce warnings when compiling swift-syntax without library evolution using Swift ≥5.10.

Replace them by `private import` when compiling using Swift ≥5.11.

Mirrors swiftlang/swift-syntax#2429

Author: keith

Sources/ArgumentParser/Usage/DumpHelpGenerator.swift

@@ -9,8 +9,13 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import ArgumentParserToolInfo
-@_implementationOnly import class Foundation.JSONEncoder
+#if swift(>=5.11)
+private import ArgumentParserToolInfo
+private import class Foundation.JSONEncoder
+#else
+import ArgumentParserToolInfo
+import class Foundation.JSONEncoder
+#endif
 
 internal struct DumpHelpGenerator {
   var toolInfo: ToolInfoV0

Sources/ArgumentParser/Usage/MessageInfo.swift

@@ -9,8 +9,13 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import protocol Foundation.LocalizedError
-@_implementationOnly import class Foundation.NSError
+#if swift(>=5.11)
+private import protocol Foundation.LocalizedError
+private import class Foundation.NSError
+#else
+import protocol Foundation.LocalizedError
+import class Foundation.NSError
+#endif
 
 enum MessageInfo {
   case help(text: String)

Sources/ArgumentParser/Usage/UsageGenerator.swift

@@ -9,7 +9,11 @@
 //
 //===----------------------------------------------------------------------===//
 
-@_implementationOnly import protocol Foundation.LocalizedError
+#if swift(>=5.11)
+private import protocol Foundation.LocalizedError
+#else
+import protocol Foundation.LocalizedError
+#endif
 
 struct UsageGenerator {
   var toolName: String