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 @@
  <IsPackableCondition="'$(PackageId)' != ''">true</IsPackable>
  </PropertyGroup>
 
+ <PropertyGroup>
+<!-- Save original PackageId to restore post-import -->
+ <_PackageId>$(PackageId)</_PackageId>
+ </PropertyGroup>
+
  <ImportProject="$(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 -->
- <PackageIdCondition="'$(IsPackable)' == 'false'"></PackageId>
+ <PackageIdCondition="'$(IsPackable)' == 'false'">$(_PackageId)</PackageId>
+<!-- If no PackageId was originally provided, ensure IsPackable is false -->
+ <IsPackableCondition="'$(_PackageId)' == ''">false</IsPackable>
  </PropertyGroup>
 
  <ItemGroupCondition="'$(IsPackable)' == 'true'"Label="NuGet">