Fix #2226 (#2249)

* fix #2226

* file scoped namespaces and minor cleanup

* use code simplification suggested by KalleOlaviNiemitalo

* ensure help is not shown when help option is not present

* correctly look up HelpAction on ancestor commands' help options

Author: jonsequitur

src/System.CommandLine.Tests/EnvironmentVariableDirectiveTests.cs

@@ -2,7 +2,6 @@
 using System.CommandLine.Invocation;
 using FluentAssertions;
 using System.Linq;
-using System.Threading;
 using System.Threading.Tasks;
 
 using Xunit;