Move UsingScope and CSharpTypeResolveContext into the type system

Both types are consumed well outside the C# output layer - DecompileRun
carries the using scope, and the IL transforms build a resolve context from
it - so living in ICSharpCode.Decompiler.CSharp.TypeSystem misrepresented
where they belong and forced a C#-specific namespace import on every
consumer.

Assisted-by: Claude:claude-opus-5[1m]:Claude Code

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -32,7 +32,6 @@ using ICSharpCode.Decompiler.CSharp.OutputVisitor;
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.CSharp.Transforms;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.DebugSteps;
 using ICSharpCode.Decompiler.DebugInfo;
 using ICSharpCode.Decompiler.Disassembler;
@@ -44,6 +43,7 @@ using ICSharpCode.Decompiler.Instrumentation;
 using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
 using SRM = System.Reflection.Metadata;

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -28,7 +28,6 @@ using System.Threading;
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.CSharp.Transforms;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.IL.Transforms;
 using ICSharpCode.Decompiler.Semantics;

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -23,7 +23,6 @@ using System.Diagnostics;
 using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -27,7 +27,6 @@ using System.Reflection.Metadata.Ecma335;
 using System.Runtime.CompilerServices;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceExtensionMethods.cs

@@ -25,9 +25,9 @@ using System.Linq;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
 using ICSharpCode.Decompiler.CSharp.Syntax;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.CSharp.Transforms

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -29,7 +29,6 @@ using ICSharpCode.Decompiler.CSharp.Syntax;
 #if STEP
 using ICSharpCode.Decompiler.CSharp.Syntax.PatternMatching;
 #endif
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.IL;
 using ICSharpCode.Decompiler.Semantics;
 using ICSharpCode.Decompiler.TypeSystem;

ICSharpCode.Decompiler/DecompileRun.cs

@@ -46,9 +46,9 @@ namespace ICSharpCode.Decompiler
 
 		public Dictionary<ITypeDefinition, bool> TypeHierarchyIsKnown { get; } = new();
 
-		public CSharp.TypeSystem.UsingScope UsingScope { get; }
+		public UsingScope UsingScope { get; }
 
-		public DecompileRun(DecompilerSettings settings, CSharp.TypeSystem.UsingScope usingScope)
+		public DecompileRun(DecompilerSettings settings, UsingScope usingScope)
 		{
 			this.Settings = settings ?? throw new ArgumentNullException(nameof(settings));
 			this.UsingScope = usingScope ?? throw new ArgumentNullException(nameof(usingScope));

ICSharpCode.Decompiler/IL/Transforms/AssignVariableNames.cs

@@ -29,7 +29,6 @@ using Humanizer.Inflections;
 using ICSharpCode.Decompiler.CSharp;
 using ICSharpCode.Decompiler.CSharp.OutputVisitor;
 using ICSharpCode.Decompiler.CSharp.Transforms;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem;
 using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;

ICSharpCode.Decompiler/IL/Transforms/IILTransform.cs

@@ -24,10 +24,10 @@ using System.Diagnostics;
 using System.Threading;
 
 using ICSharpCode.Decompiler.CSharp.Resolver;
-using ICSharpCode.Decompiler.CSharp.TypeSystem;
 using ICSharpCode.Decompiler.DebugSteps;
 using ICSharpCode.Decompiler.DebugInfo;
 using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
 namespace ICSharpCode.Decompiler.IL.Transforms

ICSharpCode.Decompiler/TypeSystem/Implementation/CSharpTypeResolveContext.cs

@@ -18,9 +18,7 @@
 
 using System;
 
-using ICSharpCode.Decompiler.TypeSystem;
-
-namespace ICSharpCode.Decompiler.CSharp.TypeSystem
+namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 {
 	public sealed class CSharpTypeResolveContext : ITypeResolveContext
 	{

ICSharpCode.Decompiler/TypeSystem/UsingScope.cs

@@ -23,12 +23,12 @@ using System.Collections.Immutable;
 
 using ICSharpCode.Decompiler.CSharp.Syntax;
 using ICSharpCode.Decompiler.Semantics;
-using ICSharpCode.Decompiler.TypeSystem;
+using ICSharpCode.Decompiler.TypeSystem.Implementation;
 using ICSharpCode.Decompiler.Util;
 
 #nullable enable
 
-namespace ICSharpCode.Decompiler.CSharp.TypeSystem
+namespace ICSharpCode.Decompiler.TypeSystem
 {
 	/// <summary>
 	/// Represents a scope that contains "using" statements.