Implicit refs error when CPM is enabled

I saw an implicit refs error when I enabled CPM. That seems bad. This means that the SDK is the only place where the version can be specified, unless [transitive pinning](https://learn.microsoft.com/nuget/consume-packages/Central-Package-Management#transitive-pinning) is used, which isn't mentioned.

> /Users/rich/git/winforms/src/System.Windows.Forms.Analyzers/tests/UnitTests/System.Windows.Forms.Analyzers.Tests.csproj : error NU1009: The packages xunit.assert are implicitly referenced. You do not typically need to reference them from your project or in your central package versions management file. For more information, see https://aka.ms/sdkimplicitrefs

https://github.com/dotnet/winforms/pull/12175

I'm not sure where the error us. It might be in our internal SDKs.

There are two questions here:

- Should we update the error message to include a reference to transitive pinning?
- Should we remove packages from implicit refs?

@JonDouglas