Clean up .Legacy

Also sealed two types that didn't get the treatment.

src/Microsoft.AspNetCore.Razor.Language/HtmlAttributeValueStyle.cs

@@ -1,8 +1,10 @@
 // Copyright (c) .NET Foundation. All rights reserved.
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
-namespace Microsoft.AspNetCore.Razor.Language.Legacy
+namespace Microsoft.AspNetCore.Razor.Language
 {
+    // This is the design time equivalent of Microsoft.AspNetCore.Razor.TagHelpers.HtmlAttributeValueStyle.
+    // They should be kept in sync.
     public enum HtmlAttributeValueStyle
     {
         DoubleQuotes,

src/Microsoft.AspNetCore.Razor.Language/Legacy/BlockKindInternal.cs

@@ -3,7 +3,7 @@
 
 namespace Microsoft.AspNetCore.Razor.Language.Legacy
 {
-    public enum BlockKindInternal
+    internal enum BlockKindInternal
     {
         // Code
         Statement,

src/Microsoft.AspNetCore.Razor.Language/Legacy/LocationTagged.cs

@@ -9,7 +9,7 @@ using Microsoft.Extensions.Internal;
 namespace Microsoft.AspNetCore.Razor.Language.Legacy
 {
     [DebuggerDisplay("({Location})\"{Value}\"")]
-    public class LocationTagged<TValue> : IFormattable
+    internal class LocationTagged<TValue> : IFormattable
     {
         public LocationTagged(TValue value, int absoluteIndex, int lineIndex, int characterIndex)
             : this (value, new SourceLocation(absoluteIndex, lineIndex, characterIndex))

src/Microsoft.CodeAnalysis.Razor/CompilationTagHelperFeature.cs

@@ -8,7 +8,7 @@ using Microsoft.CodeAnalysis.CSharp;
 
 namespace Microsoft.CodeAnalysis.Razor
 {
-    public class CompilationTagHelperFeature : RazorEngineFeatureBase, ITagHelperFeature
+    public sealed class CompilationTagHelperFeature : RazorEngineFeatureBase, ITagHelperFeature
     {
         private ITagHelperDescriptorProvider[] _providers;
         private IMetadataReferenceFeature _referenceFeature;

src/Microsoft.CodeAnalysis.Razor/DefaultMetadataReferenceFeature.cs

@@ -6,7 +6,7 @@ using Microsoft.AspNetCore.Razor.Language;
 
 namespace Microsoft.CodeAnalysis.Razor
 {
-    public class DefaultMetadataReferenceFeature : RazorEngineFeatureBase, IMetadataReferenceFeature
+    public sealed class DefaultMetadataReferenceFeature : RazorEngineFeatureBase, IMetadataReferenceFeature
     {
         public IReadOnlyList<MetadataReference> References { get; set; }
     }