Move global.json and nuget.config to samples dir

We don't need to change these settings repository-wide. They can be
set for only the `samples` directory.

`Xamarin.Legacy.Sdk` itself can be built without any fancy
configuration.

global.json

@@ -1,11 +1,5 @@
 {
-  "sdk": {
-    "version": "6.0.100-alpha.1.21064.27",
-    "rollForward": "disable",
-    "allowPrerelease": true
-  },
   "msbuild-sdks": {
-    "Microsoft.Build.NoTargets": "2.0.1",
-    "Xamarin.Legacy.Sdk": "0.1.0-alpha1"
+    "Microsoft.Build.NoTargets": "2.0.1"
   }
 }

samples/global.json

@@ -0,0 +1,10 @@
+{
+  "sdk": {
+    "version": "6.0.100-alpha.1.21064.27",
+    "rollForward": "disable",
+    "allowPrerelease": true
+  },
+  "msbuild-sdks": {
+    "Xamarin.Legacy.Sdk": "0.1.0-alpha1"
+  }
+}

samples/nuget.config

@@ -2,13 +2,13 @@
 <configuration>
   <packageSources>
     <clear />
-    <add key="debug"   value="bin/Debug/" />
-    <add key="release" value="bin/Release/" />
+    <add key="debug"   value="../bin/Debug/" />
+    <add key="release" value="../bin/Release/" />
     <add key="dotnet6" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet6/nuget/v3/index.json" />
-    <add key="xamarin-impl" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/xamarin-impl/nuget/v3/index.json" />
+    <add key="xamarin" value="https://pkgs.dev.azure.com/azure-public/vside/_packaging/xamarin-impl/nuget/v3/index.json" />
     <add key="nuget"   value="https://api.nuget.org/v3/index.json" />
   </packageSources>
   <config>
-    <add key="globalPackagesFolder" value="packages" />
+    <add key="globalPackagesFolder" value="../packages" />
   </config>
 </configuration>