Add alternative way to specify mvc/rcl area

[iw-stefro]

We're using the areas functionality in a large project and it seems as in ASP.NET Core we need to have the areas folder structure in place on disk in each library to make it work.

In earlier MVC/Razor version I think this was a publish/deploy task to get the different libraries in the correct structure but now we must have the folder structure 'Areas/Area/Views' for all projects on disk.

To me the additional 'Areas/Area' folders add unneccesary complexity and makes code browsing/searching harder

Suggestion:

• Add a possibility to specify the area in the sdk project file.
  New setting similar to existing, see https://learn.microsoft.com/en-us/aspnet/core/razor-pages/sdk

(I've tried to do something like this with the @namespace directive but I get AspNetCoreGeneratedDocument.* names which don't work..)