Random way of building startup...

BehindTheScenes.cs

public static class WebHostBuilderExtensions
{
  public static WebHostBuilder UseProspero(this WebHostBuilder builder, Action<ProsperoFeatures> featuresAction)
  {
    builder
      .UseKestrel()
      .UseContentRoot(Directory.GetCurrentDirectory())
      .UseIISIntegration();
      
    builder.UseConventionAutofacServiceProvider(); // Finds Module subtypes
    
    var features = new ProsperoFeatures();
    featuresAction(features);
    
    var featureInstaller = new ProsperoFeatureInstaller(features);
    featureInstaller.Install(builder);
  }
}

public class ProsperoFeatures
{
    public IList<Assembly> Assemblies { get; }
    
    public IList<Action<IServiceCollection>> ServiceConfigurations {get;}
    public IList<Action<SomeContainerBuilder>> ContainerBuilderActions {get;}
    public IList<Action<IApplicationBuilder>> ApplicationBuilderActions {get;}
}

Program.cs

public class Program
{
  public static void Main(string[] args)
  {
    var host = new WebHostBuilder()
      .UseProspero(
        features => 
          features
            .AddAssemblyContaining<CoreItem>
            .AddAssemblyContaining<Startup>
            .UseLogging(log => log.AddDebug())
            .UseImplementations(imp => imp.WithAllInterfaces())
            .UseAutomapper()
            .UseServiceBus(sb => sb.WithSqlServer())
            .UseCoolDeveloperShit()
            .UseBoringUserShit()
            .UseMvc()
      )
      .UseStartup<Startup>()
      .Build();
      host.Run();
  }
  
  public class Startup
  {
    public void ConfigureContainer(WhateverContainerWeAgreeOn container)
    {
      container.RegisterMyApplicationSpecificStuff();
    }
    
    // Alternatively
    public void ConfigureServices(IServiceCollection services)
    {
      services.AddScoped<MyService>();
    }
  }
}