Unify @argument and @option initialization paths (#477)

- Fixes #466.
- Adds initializers to ArgumentDefinition generic over a Container type.
  The Container type must conform to a new internal protocol
  ArgumentDefinitionContainer which describes functionality like default
  set of help options for the argument defined by the property wrapper,
  etc.
- Adds overloads for Optional @arguments and @options with default
  values which emit deprecation warning to guide users towards using the
  non-Optional versions.

Author: rauhul

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -400,7 +400,13 @@ extension Flag where Value: EnumerableFlag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Value.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: initial != nil ? .isOptional : [], help: helpForCase, defaultValue: defaultValue, key: key, isComposite: !hasCustomCaseHelp)
+        let help = ArgumentDefinition.Help(
+          allValues: [],
+          options: initial != nil ? [.isOptional] : [],
+          help: helpForCase,
+          defaultValue: defaultValue,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: initial, update: .nullary({ (origin, name, values) in
           hasUpdated = try ArgumentSet.updateFlag(key: key, value: value, origin: origin, values: &values, hasUpdated: hasUpdated, exclusivity: exclusivity)
         }))
@@ -489,7 +495,15 @@ extension Flag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: .isOptional, help: helpForCase, key: key, isComposite: !hasCustomCaseHelp)
+
+        let help = ArgumentDefinition.Help(
+          allValues: [],
+          options: [.isOptional],
+          help: helpForCase,
+          defaultValue: nil,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
+
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: nil as Element?, update: .nullary({ (origin, name, values) in
           hasUpdated = try ArgumentSet.updateFlag(key: key, value: value, origin: origin, values: &values, hasUpdated: hasUpdated, exclusivity: exclusivity)
         }))
@@ -514,7 +528,14 @@ extension Flag {
         let caseKey = InputKey(rawValue: String(describing: value))
         let name = Element.name(for: value)
         let helpForCase = hasCustomCaseHelp ? (caseHelps[i] ?? help) : help
-        let help = ArgumentDefinition.Help(options: .isOptional, help: helpForCase, key: key, isComposite: !hasCustomCaseHelp)
+        let help = ArgumentDefinition.Help(
+          allValues: [],
+          options: [.isOptional],
+          help: helpForCase,
+          defaultValue: nil,
+          key: key,
+          isComposite: !hasCustomCaseHelp)
+
         return ArgumentDefinition.flag(name: name, key: key, caseKey: caseKey, help: help, parsingStrategy: .default, initialValue: initial, update: .nullary({ (origin, name, values) in
           values.update(forKey: key, inputOrigin: origin, initial: [Element](), closure: {
             $0.append(value)