Require package readmes for shipping packages

[ViktorHofer]

Contributes to #99358

1. Commit: Require package readmes for shipping packages
   Add a target that validates that a package readme
   exists for a shipping package.
   Set EnableDefaultPackageReadmeFile=false in projects
   that don't yet have a package readme.
2. Commit: Delete empty package readme files that were accidentally committed

I will send out a mail to the runtimerepo DL after this change is merged to explain that package readmes are now required and
to create energy on adding the remaining 30 package READMEs, ideally ready for .NET 9.

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[am11]

I noticed that the packs subset also reports some missing readmes:

2024-07-15T09:43:53.9976756Z   The package Microsoft.NETCore.App.Ref.9.0.0-dev is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
2024-07-15T09:43:53.9976756Z   The package Microsoft.NETCore.App.Ref.9.0.0-dev is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
2024-07-15T09:44:32.1884036Z   The package Microsoft.NETCore.App.Crossgen2.freebsd-x64.9.0.0-dev is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
2024-07-15T09:44:55.9404147Z   The package Microsoft.NETCore.App.Runtime.freebsd-x64.9.0.0-dev is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.
2024-07-15T09:45:17.6289662Z   The package Microsoft.NET.HostModel.9.0.0-dev is missing a readme. Go to https://aka.ms/nuget/authoring-best-practices/readme to learn why package readmes are important.

[ViktorHofer]

@am11 I changed a property so that this validation is only enabled for src/libraries packages (for now).

[am11]

Sounds good. I think the validation is probably hitting false-positive, because those are internal packages and nuget.org shows this line:

runtime/Directory.Build.targets

Line 77 in 66ae90f

<RuntimePackageDisclaimer>Internal implementation package not meant for direct consumption. Please do not reference directly.</RuntimePackageDisclaimer>

e.g. https://www.nuget.org/packages/Microsoft.NETCore.App.Crossgen2.linux-x64. In these cases, we probably don't need a physical readme file.

[ViktorHofer]

Sounds good. I think the validation is probably hitting false-positive, because those are internal packages and nuget.org shows this line:

Yes, we can revisit this when we want to expand the validation to other packages produced in runtime, outside of src/libraries.

[ViktorHofer]

@dotnet/runtime-infrastructure, @ericstj would you mind reviewing?

[carlossanlop]

Looks good. I have questions.

[carlossanlop]

LGTM. Consider my comment suggestions optional. Feel free to wait for a review from Eric too.