Improve failure message for backdeployed AsyncParsableCommand (#547)

When an executable with asynchronous commands is backdeployed,
the compiler chooses the synchronous `main()` unless a minimum
availability target is provided for the root command type. This
changes the error message provided when the incorrect `main()`
function is called to direct the tool's author to a correct
solution.

Author: natecook1000

Sources/ArgumentParser/Parsable Types/ParsableCommand.swift

@@ -127,7 +127,13 @@ extension ParsableCommand {
 
 #if DEBUG
     if #available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *) {
-      checkAsyncHierarchy(self, root: "\(self)")
+      if let asyncCommand = firstAsyncSubcommand(self) {
+        if Self() is AsyncParsableCommand {
+          failAsyncPlatform(rootCommand: self)
+        } else {
+          failAsyncHierarchy(rootCommand: self, subCommand: asyncCommand)
+        }
+      }
     }
 #endif
 
@@ -194,5 +200,57 @@ extension ParsableCommand {
       """.wrapped(to: 70))
     }
   }
+
+  @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
+  internal static func firstAsyncSubcommand(_ command: ParsableCommand.Type) -> AsyncParsableCommand.Type? {
+    for sub in command.configuration.subcommands {
+      if let asyncCommand = sub as? AsyncParsableCommand.Type,
+         sub.configuration.subcommands.isEmpty
+      {
+        return asyncCommand
+      }
+      
+      if let asyncCommand = firstAsyncSubcommand(sub) {
+        return asyncCommand
+      }
+    }
+    
+    return nil
+  }
 #endif
 }
+
+// MARK: Async Configuration Errors
+
+func failAsyncHierarchy(
+  rootCommand: ParsableCommand.Type, subCommand: ParsableCommand.Type
+) -> Never {
+  fatalError("""
+
+  --------------------------------------------------------------------
+  Asynchronous subcommand of a synchronous root.
+
+  The asynchronous command `\(subCommand)` is declared as a subcommand of the synchronous root command `\(rootCommand)`.
+
+  With this configuration, your asynchronous `run()` method will not be called. To fix this issue, change `\(rootCommand)`'s `ParsableCommand` conformance to `AsyncParsableCommand`.
+  --------------------------------------------------------------------
+
+  """.wrapped(to: 70))
+}
+
+func failAsyncPlatform(rootCommand: ParsableCommand.Type) -> Never {
+  fatalError("""
+
+  --------------------------------------------------------------------
+  Asynchronous root command needs availability annotation.
+
+  The asynchronous root command `\(rootCommand)` needs an availability annotation in order to be executed asynchronously. To fix this issue, add the following availability attribute to your `\(rootCommand)` declaration or set the minimum platform in your "Package.swift" file.
+  
+  """.wrapped(to: 70)
+  + """
+  
+  @available(macOS 10.15, macCatalyst 13, iOS 13, tvOS 13, watchOS 6, *)
+  --------------------------------------------------------------------
+  
+  """)
+}