Remove redudnant internals visible to in ms test.source generation

[SimonCropp]

No description provided.

[Evangelink]

I am not a super big fan of having public types when it's not needed - we have too many issues and problems linked to public types where we don't know if they are used preventing us from doing any change.

I also think IVTs kinda suck but from source to test it feels ok to me - at least preferable than public.

[nohwnd]

Keeping internal, and replacing by #5561

[SimonCropp]

@Evangelink but isnt this a source generator package, and hence not possible for people to reference its types as a public api

[Youssef1313]

@Evangelink but isnt this a source generator package, and hence not possible for people to reference its types as a public api

It's technically possible to reference the types. But it requires adding an explicit reference in csproj similar to:

<Reference Include="$(NuGetPackageRoot)\package.name\package.version\analyzers\dotnet\cs\FileName.dll" />

It's extremely uncommon though, and I personally wouldn't consider it as a supported scenario. Still, I would like to keep IVT for tests once the work for polyfill EmbeddedAttribute is done. I haven't yet got to complete my PR though, but hopefully soon I'll complete it.