source-build: support building runtime using non-portable runtime packages (backport of #75597) (#77509)

Author: ayakael

Directory.Build.targets

@@ -11,6 +11,21 @@
   <Import Project="$(RepositoryEngineeringDir)liveBuilds.targets" />
   <Import Project="$(RepositoryEngineeringDir)python.targets" />
 
+  <!--
+  When .NET gets built from source, make the SDK aware there are bootstrap packages
+  for Microsoft.NETCore.App.Runtime.<rid> and Microsoft.NETCore.App.Crossgen2.<rid>.
+  -->
+  <ItemGroup Condition="'$(DotNetBuildFromSource)' == 'true'">
+    <KnownFrameworkReference Update="@(KnownFrameworkReference->WithMetadataValue('Identity', 'Microsoft.NETCore.App')->WithMetadataValue('TargetFramework', '$(NetCoreAppCurrent)'))">
+      <RuntimePackRuntimeIdentifiers>$(PackageRID)</RuntimePackRuntimeIdentifiers>
+    </KnownFrameworkReference>
+    <KnownCrossgen2Pack Update="@(KnownCrossgen2Pack->WithMetadataValue('Identity', 'Microsoft.NETCore.App.Crossgen2')->WithMetadataValue('TargetFramework', '$(NetCoreAppCurrent)'))">
+      <Crossgen2RuntimeIdentifiers>$(PackageRID)</Crossgen2RuntimeIdentifiers>
+    </KnownCrossgen2Pack>
+    <!-- Avoid references to Microsoft.AspNetCore.App.Runtime.<rid> -->
+    <KnownFrameworkReference Remove="Microsoft.AspNetCore.App" />
+  </ItemGroup>
+
   <PropertyGroup>
     <!--
       Define this here (not just in Versions.props) because the SDK resets it