Add razor redirector implementation

Author: chsienki

src/Razor/src/Microsoft.CodeAnalysis.Razor.CohostingShared/Microsoft.CodeAnalysis.Razor.CohostingShared.projitems

@@ -32,6 +32,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Formatting\CohostRangeFormattingEndpoint.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)OnAutoInsert\CohostOnAutoInsertEndpoint.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)OnAutoInsert\CohostOnAutoInsertTriggerCharacterProviders.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)RazorAnalyzerAssemblyRedirector.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Rename\CohostRenameEndpoint.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)SignatureHelp\CohostSignatureHelpEndpoint.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)FoldingRange\CohostFoldingRangeEndpoint.cs" />

src/Razor/src/Microsoft.CodeAnalysis.Razor.CohostingShared/RazorAnalyzerAssemblyRedirector.cs

@@ -0,0 +1,56 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the MIT license. See License.txt in the project root for license information.
+
+using System;
+using System.Collections.Immutable;
+using System.IO;
+using System.Reflection;
+using System.Composition;
+using IRazorAnalyzerAssemblyRedirector = Microsoft.CodeAnalysis.ExternalAccess.Razor.RazorAnalyzerAssemblyRedirector.IRazorAnalyzerAssemblyRedirector;
+
+namespace Microsoft.VisualStudio.Razor;
+
+#pragma warning disable RS0030 // Do not use banned APIs
+[Shared]
+[Export(typeof(IRazorAnalyzerAssemblyRedirector))]
+[method: ImportingConstructor]
+#pragma warning restore RS0030 // Do not use banned APIs
+internal class RazorAnalyzerAssemblyRedirector() : IRazorAnalyzerAssemblyRedirector
+{
+    private static readonly ImmutableArray<Type> s_compilerAssemblyTypes = [
+
+        typeof(CodeAnalysis.Razor.CompilerFeatures), // Microsoft.CodeAnalysis.Razor.Compiler.dll
+        typeof(AspNetCore.Razor.ArgHelper), // Microsoft.AspNetCore.Razor.Utilities.Shared.dll
+
+        // The following dependencies will be provided by the Compiler ALC so its not strictly required to redirect them, but we do so for completeness. 
+        typeof(Microsoft.Extensions.ObjectPool.ObjectPool), // Microsoft.Extensions.ObjectPool.dll
+        typeof(ImmutableArray) // System.Collections.Immutable.dll
+    ];
+
+    private static readonly ImmutableDictionary<string, string> s_compilerAssemblyMap = s_compilerAssemblyTypes.ToImmutableDictionary(t => t.Assembly.GetName().Name!, t => GetAssemblyLocation(t.Assembly));
+
+    public string? RedirectPath(string fullPath)
+    {
+        var name = Path.GetFileNameWithoutExtension(fullPath);
+        return s_compilerAssemblyMap.TryGetValue(name, out var path) ? path : null;
+    }
+
+    private static string GetAssemblyLocation(Assembly assembly)
+    {
+        var location = assembly.Location;
+        var name = Path.GetFileName(location);
+        var directory = Path.GetDirectoryName(location) ?? "";
+
+        // In VS on windows, depending on who wins the race to load these assemblies, the base directory will either be the tooling root (if Roslyn wins)
+        // or the ServiceHubCore subfolder (razor). In the root directory these are netstandard2.0 targeted, in ServiceHubCore they are .NET targeted.
+        // We need to always pick the same set of assemblies regardless of who causes us to load. Because this code only runs in a .NET based host,
+        // we want to prefer the .NET targeted ServiceHubCore versions if they exist.
+        var serviceHubCoreVersion = Path.Combine(directory, "ServiceHubCore", name);
+        if (File.Exists(serviceHubCoreVersion))
+        {
+            return serviceHubCoreVersion;
+        }
+
+        return location;
+    }
+}