Name based lookup fixes (#2255)

* fix parent lookups for GetValue(string), add GetResult(string)

* remove dead code and other cleanups

* split out CustomParsingTests

* make CliCommand IEnumerable, not IEnumerable<T>

* remove EditorBrowsable(Never) from CliCommand.Add overloads

* add custom parsing tests to cover SymbolResult.GetResult by name

Author: jonsequitur

src/System.CommandLine.ApiCompatibility.Tests/ApiCompatibilityApprovalTests.System_CommandLine_api_is_not_changed.approved.txt

@@ -35,7 +35,7 @@ System.CommandLine
     public System.Void AcceptLegalFileNamesOnly()
     public System.Void AcceptLegalFilePathsOnly()
     public System.Void AcceptOnlyFromAmong(System.String[] values)
-  public class CliCommand : CliSymbol, System.Collections.Generic.IEnumerable<CliSymbol>, System.Collections.IEnumerable
+  public class CliCommand : CliSymbol, System.Collections.IEnumerable
     .ctor(System.String name, System.String description = null)
     public System.CommandLine.Invocation.CliAction Action { get; set; }
     public System.Collections.Generic.ICollection<System.String> Aliases { get; }
@@ -49,7 +49,6 @@ System.CommandLine
     public System.Void Add(CliOption option)
     public System.Void Add(CliCommand command)
     public System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem> GetCompletions(System.CommandLine.Completions.CompletionContext context)
-    public System.Collections.Generic.IEnumerator<CliSymbol> GetEnumerator()
     public ParseResult Parse(System.Collections.Generic.IReadOnlyList<System.String> args, CliConfiguration configuration = null)
     public ParseResult Parse(System.String commandLine, CliConfiguration configuration = null)
     public System.Void SetAction(System.Action<ParseResult> action)
@@ -89,17 +88,15 @@ System.CommandLine
     public System.Boolean Recursive { get; set; }
     public System.Boolean Required { get; set; }
     public System.Collections.Generic.List<System.Action<System.CommandLine.Parsing.OptionResult>> Validators { get; }
-    public System.Type ValueType { get; }
     public System.Collections.Generic.IEnumerable<System.CommandLine.Completions.CompletionItem> GetCompletions(System.CommandLine.Completions.CompletionContext context)
   public class CliOption<T> : CliOption
     .ctor(System.String name, System.String[] aliases)
     public Func<System.CommandLine.Parsing.ArgumentResult,T> CustomParser { get; set; }
     public Func<System.CommandLine.Parsing.ArgumentResult,T> DefaultValueFactory { get; set; }
-    public System.Type ValueType { get; }
     public System.Void AcceptLegalFileNamesOnly()
     public System.Void AcceptLegalFilePathsOnly()
     public System.Void AcceptOnlyFromAmong(System.String[] values)
-  public class CliRootCommand : CliCommand, System.Collections.Generic.IEnumerable<CliSymbol>, System.Collections.IEnumerable
+  public class CliRootCommand : CliCommand, System.Collections.IEnumerable
     public static System.String ExecutableName { get; }
     public static System.String ExecutablePath { get; }
     .ctor(System.String description = )
@@ -290,8 +287,10 @@ System.CommandLine.Parsing
     public CommandResult GetResult(System.CommandLine.CliCommand command)
     public OptionResult GetResult(System.CommandLine.CliOption option)
     public DirectiveResult GetResult(System.CommandLine.CliDirective directive)
+    public SymbolResult GetResult(System.String name)
     public T GetValue<T>(CliArgument<T> argument)
     public T GetValue<T>(CliOption<T> option)
+    public T GetValue<T>(System.String name)
   public delegate TryReplaceToken : System.MulticastDelegate, System.ICloneable, System.Runtime.Serialization.ISerializable
     .ctor(System.Object object, System.IntPtr method)
     public System.IAsyncResult BeginInvoke(System.String tokenToReplace, ref System.Collections.Generic.IReadOnlyList<System.String> replacementTokens, ref System.String& errorMessage, System.AsyncCallback callback, System.Object object)