Mistakes generate winmdroot when Windows.Win32 not generated

Actual behavior

When generate OverloadResolutionPriorityAttribute, it will add using winmdroot = global::Windows.Win32;:

// ------------------------------------------------------------------------------
// <auto-generated>
//     This code was generated by a tool.
//
//     Changes to this file may cause incorrect behavior and will be lost if
//     the code is regenerated.
// </auto-generated>
// ------------------------------------------------------------------------------

#pragma warning disable CS1591,CS1573,CS0465,CS0649,CS8019,CS1570,CS1584,CS1658,CS0436,CS8981,SYSLIB1092
using global::System;
using global::System.Diagnostics;
using global::System.Diagnostics.CodeAnalysis;
using global::System.Runtime.CompilerServices;
using global::System.Runtime.InteropServices;
using winmdroot = global::Windows.Win32;
namespace System.Runtime.CompilerServices
{
	/// <summary>
	/// Specifies the priority of a member in overload resolution.
	/// When unspecified, the default priority is 0.
	/// </summary>
	[global::System.AttributeUsage(global::System.AttributeTargets.Constructor | global::System.AttributeTargets.Method | global::System.AttributeTargets.Property, AllowMultiple = false, Inherited = false)]
	internal sealed class OverloadResolutionPriorityAttribute 		: global::System.Attribute
	{
		/// <summary>
		/// Initializes a new instance of the <see cref="OverloadResolutionPriorityAttribute"/> class.
		/// </summary>
		/// <param name="priority">The priority of the attributed member. Higher numbers are prioritized, lower numbers are deprioritized. 0 is the default if no attribute is present.</param>
		public OverloadResolutionPriorityAttribute(int priority)

		{
			this.Priority = priority;
		}


		/// <summary>
		/// The priority of the member.
		/// </summary>
		public int Priority 		{
			get; 		}
	}
}

But when we do not generate any things under Windows.Win32, it will throw C:\Users\qq251\Downloads\GitHub\DotNetHost.Win32Metadata\DotNetHost.Win32Metadata.Test\obj\Debug\net4.8.1\Microsoft.Windows.CsWin32\Microsoft.Windows.CsWin32.SourceGenerator\Windows.Win32.OverloadResolutionPriorityAttribute.g.cs(16,27,16,34): error CS0400: 未能在全局命名空间中找到类型或命名空间名“Windows”(是否缺少程序集引用?)

Expected behavior

Do not generate using winmdroot = global::Windows.Win32; when not use it.

Repro steps

Use any 3d-part metadata which does not use any member under Windows.Win32.

Context

• CsWin32 version: 0.3.269
• Target Framework: net4.8.1
• LangVersion (if explicitly set by project): latest

Reproduce the issue with 3d-part metadata that uses no members under Windows.Win32, then inspect generation of Windows.Win32.OverloadResolutionPriorityAttribute.g.cs in the Microsoft.Windows.CsWin32.SourceGenerator output. The fix is complete when the generated file omits the winmdroot alias and the project compiles without the missing Windows namespace error.