Updated Pipeline to use PipelinePhases

* Removed nested Pipelin.Subsystems
* Removed tests due to immutable ValueSubsystem
* Fixed bugs found in testing:
  * Kind needs to be passed to PipelinePhase constructor, Subsystem may be null so is not userful for retrieving
  * Phase list needs to be added in Pipeline constructor so is available to empty pipeline
  * PipelinePhrase needs to be a reference type as changes need to be available
  * Logic for getting specific subsystems didn't account for nulls correclty
* Teardown now consistently runs Execute
* Invocation and Validation are phases

Author: KathleenDollard

src/System.CommandLine.Subsystems.Tests/ValueSubsystemTests.cs

@@ -29,7 +29,7 @@ public void ValueSubsystem_is_activated_by_default()
 
         isActive.Should().BeTrue();
     }
-
+    /* Hold these tests until we determine if ValueSubsystem is replaceable
     [Fact]
     public void ValueSubsystem_returns_values_that_are_entered()
     {
@@ -90,4 +90,5 @@ public void ValueSubsystem_returns_calculated_default_value_when_no_value_is_ent
 
         pipeline.Value.GetValue<int>(option).Should().Be(expected);
     }
+    */
 }