Ingest and import Windows Forms analyzer props

Provide a mechanism to ingest Windows Forms specific analyzer props and
targets into Microsoft.NET.Sdk.WindowsDesktop from Windows Forms transport
package, and copy these files into `targets` folder of the SDK, so that
the props files are resolved and imported when a developer builds a Windows
Forms app.

In the end the new props file will end up in a location similar to this:
`C:\Program Files\dotnet\sdk\6.0.<version>\Sdks\Microsoft.NET.Sdk.WindowsDesktop\targets\`.

This work relates to dotnet/designs#223, and acts
as a workaround until dotnet/designs#181 is
approved and implemented.

packaging/Microsoft.NET.Sdk.WindowsDesktop/Microsoft.NET.Sdk.WindowsDesktop.ArchNeutral.csproj

@@ -1,4 +1,4 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <ProjectGuid>{440d06b8-e3de-4c0d-ad25-cd4f43d836e1}</ProjectGuid>
     <TargetFramework>net6.0</TargetFramework>
@@ -25,4 +25,30 @@
     <PackagingContent Include="targets\*" SubFolder="root\targets" />
     <PackagingContent Include="useSharedDesignerContext.txt" SubFolder="root" />
   </ItemGroup>
+
+  <!-- Windows Forms specific -->
+  <PropertyGroup>
+    <_WinFormsNuGetPath>$(PkgMicrosoft_Private_Winforms)</_WinFormsNuGetPath>
+    <_WinFormsContentPath>$(_WinFormsNuGetPath)\sdk\dotnet-wpf\*</_WinFormsContentPath>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackagingContent Include="$(_WinFormsContentPath)" SubFolder="root\targets" />
+  </ItemGroup>
+
+  <!-- 
+    This target *only* validates the content of Microsoft.Private.Winforms NuGet package
+    to ensure we correctly import and reference props and targets.
+   -->
+  <Target Name="_EnsureWindowsFormsPackagingContent" BeforeTargets="IdentifyPackageAssets">
+    <Error Text="Unable to resolve path to Microsoft.Private.Winforms NuGet package. Is %24(PkgMicrosoft_Private_Winforms) defined?"
+           Condition="'$(_WinFormsNuGetPath)' == ''"/>
+
+    <ItemGroup>
+      <_WinFormsContent Include="$(_WinFormsContentPath)" />
+    </ItemGroup>
+
+    <Error Text="Microsoft.Private.Winforms NuGet package contains no content. Is this expected?"
+           Condition="@(_WinFormsContent->Count()) == 0"/>
+  </Target>
 </Project>

packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.props

@@ -161,4 +161,7 @@
     <SupportedTargetFramework Remove="@(_UnsupportedNETCoreAppTargetFramework);@(_UnsupportedNETStandardTargetFramework);@(_UnsupportedNETFrameworkTargetFramework)" />
   </ItemGroup>
 
+  <!-- Import Windows Forms specific props -->
+  <Import Project="System.Windows.Forms.Analyzers.props" />
+
 </Project>

packaging/Microsoft.NET.Sdk.WindowsDesktop/targets/Microsoft.NET.Sdk.WindowsDesktop.targets

@@ -155,4 +155,7 @@
   <!-- Import WPF Build logic only when we don't import NETFX's WinFX targets -->
   <Import Project="Microsoft.WinFX.targets" Condition="'$(ImportFrameworkWinFXTargets)' != 'true'"/>
 
+  <!-- Import Windows Forms specific targets -->
+  <Import Project="System.Windows.Forms.Analyzers.targets" />
+
 </Project>