Warn if SymbolPackageFormat set to snupkg

To avoid the repeat of dotnet/extensions#4406,
warn if SymbolPackageFormat property is set to 'snupkg'.

If necessary, the warning can be suppressed with SupressSymbolPackageFormatValidation=true.

src/Microsoft.DotNet.Arcade.Sdk/sdk/Sdk.targets

@@ -23,6 +23,8 @@
     </ItemGroup>
   </Target>
 
+  <Import Project="..\tools\Pack.targets" Condition="'$(__ImportPackTargets)' == 'true'" />
+
   <Import Project="..\tools\Imports.targets" Condition="!$(_SuppressSdkImports) and !$(_SuppressAllTargets)" />
   <Import Project="..\tools\Empty.targets" Condition="!$(_SuppressSdkImports) and $(_SuppressAllTargets)"/>
 </Project>

src/Microsoft.DotNet.Arcade.Sdk/tools/Build.proj

@@ -175,6 +175,7 @@
     <ItemGroup>
       <_SolutionBuildProps Include="@(_CommonProps)"/>
       <_SolutionBuildProps Include="__DeployProjectOutput=$(Deploy)" Condition="'$(Deploy)' != ''"/>
+      <_SolutionBuildProps Include="__ImportPackTargets=true" Condition="'$(Pack)' == 'true'" />
     </ItemGroup>
 
     <!--

src/Microsoft.DotNet.Arcade.Sdk/tools/Pack.targets

@@ -0,0 +1,16 @@
+<!-- Licensed to the .NET Foundation under one or more agreements. The .NET Foundation licenses this file to you under the MIT license. -->
+<Project>
+
+  <PropertyGroup>
+    <GenerateNuspecDependsOn>$(GenerateNuspecDependsOn); _ValidationSymbolPackageFormat</GenerateNuspecDependsOn>
+  </PropertyGroup>
+
+  <Target
+      Name="_ValidationSymbolPackageFormat"
+      Condition="'$(IsPackable)' == 'true' AND '$(SupressSymbolPackageFormatValidation)' != 'true'">
+
+    <Error Text="&lt;SymbolPackageFormat&gt;snupkg&lt;/SymbolPackageFormat&gt; is not supported by the Arcade SDK."
+           Condition="'$(SymbolPackageFormat)' == 'snupkg'" /> 
+  </Target>
+
+</Project>