Razor SDK shouldn't generate the MVC recompilation attributes unless its necessary

[rynowak]

Describe the bug

A web SDK project that targets netcoreapp3.0 with no razor files will unconditionally have these attributes:

[assembly: Microsoft.AspNetCore.Mvc.ApplicationParts.RelatedAssemblyAttribute("SampleApp.Views")]
[assembly: Microsoft.AspNetCore.Razor.Hosting.RazorLanguageVersionAttribute("3.0")]
[assembly: Microsoft.AspNetCore.Razor.Hosting.RazorConfigurationNameAttribute("MVC-3.0")]
[assembly: Microsoft.AspNetCore.Razor.Hosting.RazorExtensionAssemblyNameAttribute("MVC-3.0", "Microsoft.AspNetCore.Mvc.Razor.Extensions")]

This adds extra runtime references to types that are probably not used.

This logic needs to be updated to consider whether you have any .cshtml files, and/or be based on some explicit gesture in the project file, or just obsoleted.

Basing this on the presence of .cshtml files could cause issues in the case will cause issues in the following cases:

1. You have no cshtml files at build time, and you are using runtime compilation when published
2. Youe application is an F#/VB project, referencing class libraries for views, and you are using runtime compilation

It's also an impactful change, because it requires evaluating an item group to evaluate a property, which means that it has to be in a target.

---

The RelatedAssemblyPartAttibute is purely for discovery of compiled views/pages and should be made conditional without reservation.

The Razor.Hosting attributes are for the settings used by runtime compilation. We could consider obsoleting these now that runtime compilation is outside the shared framework. However - we need to consider the impact on downlevel. If this logic is inside the Razor.Design package in 2.1/2.2 we can remove it from the SDK and say that it's not needed/supported in 3.0.

[rynowak]

/cc @pranavkm

[mkArtakMSFT]

Closing this based on the work happening in the linked issue, where we will introduce a dedicated flag on the project level, which will indicate whether those attributes should be generated or not.

[rynowak]

That doesn't address this issue.

[pranavkm]

This is needed in preview4 (at least with the current state of things).The WebSDK specifies AddRazorSupportForMvc. Some Blazor templates reference the WebSDK, so you now get compilation errors stating it can't find Mvc.