On run package validation for CSProj-built packages

This fixes an issue where a non-CSProj built package would load
MS.CA.dll , then a CSProj would try to load MS.CA.CS.dll of a different
version and it's MS.CA types would not agree with those already bound
to the task from previous load.

Further details:
dotnet/sdk#22277

Author: ericstj

eng/packaging.targets

@@ -2,12 +2,11 @@
 
   <PropertyGroup>
     <!--
-      Package validation causes flaky errors during the build. Validation isn't useful during
-      source-build (it is only a guardrail for devs), so disable it during source-build to avoid the
-      impact on build reliability. https://github.com/dotnet/runtime/issues/60883 tracks fixing the
-      flakiness and removing this condition.
+      Disable PackageValidation for non-CSProj since those will load only one roslyn assembly of the wrong
+      version causing the task to be in torn-state.  See https://github.com/dotnet/runtime/pull/61697.
+      https://github.com/dotnet/runtime/issues/60883 tracks fixing this in 6.0 and removing this condition.
     -->
-    <EnablePackageValidation Condition="'$(DotNetBuildFromSource)' != 'true'">true</EnablePackageValidation>
+    <EnablePackageValidation Condition="'$(MSBuildProjectExtension)' == '.csproj'">true</EnablePackageValidation>
     <!-- Don't restore prebuilt packages during sourcebuild. -->
     <DisablePackageBaselineValidation Condition="'$(DotNetBuildFromSource)' == 'true'">true</DisablePackageBaselineValidation>
     <PackageValidationBaselineVersion Condition="'$(PackageValidationBaselineVersion)' == ''">6.0.0</PackageValidationBaselineVersion>

src/libraries/System.IO.Ports/pkg/runtime.native.System.IO.Ports.proj

@@ -6,8 +6,6 @@
     <NoTargetsDoNotReferenceOutputAssemblies>false</NoTargetsDoNotReferenceOutputAssemblies>
     <!-- This is a meta package and doesn't contain any libs. -->
     <NoWarn>$(NoWarn);NU5128</NoWarn>
-    <!-- It doesn't make sense to run package validation on native packages -->
-    <EnablePackageValidation>false</EnablePackageValidation>
   </PropertyGroup>
 
   <ItemGroup>

src/libraries/System.IO.Ports/pkg/runtime.native.System.IO.Ports.props

@@ -13,8 +13,6 @@
     <UseRuntimePackageDisclaimer>true</UseRuntimePackageDisclaimer>
     <!-- This is a native package and doesn't contain any ref/lib assets. -->
     <NoWarn>$(NoWarn);NU5128</NoWarn>
-    <!-- It doesn't make sense to run package validation on native packages -->
-    <EnablePackageValidation>false</EnablePackageValidation>
   </PropertyGroup>
 
   <ItemGroup>