Add error for incorrect PublishSelfContained

[sbomer]

For the following project (note it sets PublishSelfContained):

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net10.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
    <PublishSelfContained>true</PublishSelfContained>
  </PropertyGroup>

</Project>

This command doesn't produce a self-contained publish output:

> dotnet build /t:Publish
Restore complete (0.4s)
You are using a preview version of .NET. See: https://aka.ms/dotnet-support-policy
  selfc succeeded (0.3s) → bin/Debug/net10.0/publish/

Build succeeded in 1.1s
> ls bin/Debug/net10.0/
publish  selfc  selfc.deps.json  selfc.dll  selfc.pdb  selfc.runtimeconfig.json
> ls bin/Debug/net10.0/publish/
selfc  selfc.deps.json  selfc.dll  selfc.pdb  selfc.runtimeconfig.json

This adds an error message for this unsupported scenario.

Related: #32272 (discusses the PublishSelfContained behavior with /t:Publish)
Related: dotnet/runtime#95496
Related: #46070

[github-actions]

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[nagilson]

Would we want this as a warning instead of an error? This will definitely cause some breaking changes but I understand the idea. We may also want to consider this for PublishRuntimeIdentifier. Its likely too late to do this for 10 but we could do this for 11.

I think we would also want sign off from @baronfel or another PM if we wanted to do this from a product perspective. I think it's a good idea.

[baronfel]

Thanks for the call-out @nagilson - definitely think there's value here, definitely agree that it's too late for 10 (our fault due to delays/time), definitely agree that it needs to be a warning, not an error, for at least a release. The error itself seems fine, it would be great if we had a help doc we could point to. I'm not sure if one exists that calls out the gaps with using /t:Publish directly.