Refactor CommandResolver into individual CommandResolver Implementation

classes.

Write Unit Tests covering Composite DefaultCommandResolver and
ScriptCommandResolver.

baseline

Baseline2

src/Microsoft.DotNet.Cli.Utils/CommandResolution/AppBaseCommandResolver.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Runtime.InteropServices;
+using Microsoft.DotNet.ProjectModel;
+using Microsoft.DotNet.ProjectModel.Graph;
+using Microsoft.Extensions.PlatformAbstractions;
+using NuGet.Frameworks;
+using NuGet.Packaging;
+namespace Microsoft.DotNet.Cli.Utils
+{
+    public class AppBaseCommandResolver : AbstractPathBasedCommandResolver
+    {
+        public AppBaseCommandResolver(IEnvironmentProvider environment, 
+            IPlatformCommandSpecFactory commandSpecFactory) : base(environment, commandSpecFactory) { }
+
+        internal override string ResolveCommandPath(CommandResolverArguments commandResolverArguments)
+        {
+            return _environment.GetCommandPathFromRootPath(
+                PlatformServices.Default.Application.ApplicationBasePath, 
+                commandResolverArguments.CommandName);
+        }
+
+        internal override CommandResolutionStrategy GetCommandResolutionStrategy()
+        {
+            return CommandResolutionStrategy.BaseDirectory;
+        }
+    }
+}