[blazor] Drop explicit change of extension of assembly to lazy load

[maraf]

• In [blazor] Use JSImport for lazy loading assemblies #46437 made an explicit change of extension of assembly to lazy load to .dll
• This change now conflicts with support for .webcil
• The assembly (file) name comes from the user and it's validated before that it points to an existing file, so the explicit change of extension is void even for .dll scenario (the user must have provided name with .dll)
• It would be better if users would be providing only assembly name, without extension at all, incuding BlazorWebAssemblyLazyLoad MSBuild item, but that is probably a breaking change

I would like to backport this change to preview4 if possible, so that webcil support is complete

[maraf]

/backport to release/8.0-preview4

[ghost]

Hi @maraf. It looks like you just commented on a closed PR. The team will most probably miss it. If you'd like to bring something important up to their attention, consider filing a new issue and add enough details to build context.

[github-actions]

Started backporting to release/8.0-preview4: https://github.com/dotnet/aspnetcore/actions/runs/4808664707