Improve PackageId/IsPackable defaulting

By preserving the project-defined PackageId and restoring it (potentially empty), we respect the project's choice. 

Also, it makes no sense for an empty PackageId to have IsPackable after pack imports+restore, so we default it back to false in that case.

Author: kzu

src/Directory.Build.targets

@@ -32,11 +32,18 @@
     <IsPackable Condition="'$(PackageId)' != ''">true</IsPackable>
   </PropertyGroup>
 
+  <PropertyGroup>
+    <!-- Save original PackageId to restore post-import -->
+    <_PackageId>$(PackageId)</_PackageId>
+  </PropertyGroup>
+
   <Import Project="$(NuGetBuildTasksPackTargets)" Condition="Exists('$(NuGetBuildTasksPackTargets)')"/>
 
   <PropertyGroup>
     <!-- Revert the forced PackageId by the NuGet SDK targets for non-packable projects, see https://github.com/NuGet/Home/issues/14928 -->
-    <PackageId Condition="'$(IsPackable)' == 'false'"></PackageId>
+    <PackageId Condition="'$(IsPackable)' == 'false'">$(_PackageId)</PackageId>
+    <!-- If no PackageId was originally provided, ensure IsPackable is false -->
+    <IsPackable Condition="'$(_PackageId)' == ''">false</IsPackable>
   </PropertyGroup>
 
   <ItemGroup Condition="'$(IsPackable)' == 'true'" Label="NuGet">