Design: Look at generic parameters when generating namespace using directives

[ajcvickers]

In various places where we generate code we look at the namespaces of the types used in order to generate using directives. This was done in #5912, which has the details of where we do this. However, if the type is generic, then we should also generate using directives for the namespaces of those types.

[yukozh]

Reproduce

Add Pomelo.EntityFrameworkCore.MySql 1.1.0 into project.json

using System;
using System.Collections.Generic;
using Microsoft.EntityFrameworkCore;

namespace ConsoleApp
{
    public class InnerModel
    {
        public string Name { get; set; }

        public int Grade { get; set; }
    }

    public class ReproduceModel
    {
        public int Id { get; set; }

        public JsonObject<List<InnerModel>> Inner { get; set; } // Requires MySQL >= 5.7
    }

    public class ReproduceContext : DbContext
    {
        public DbSet<ReproduceModel> ReproduceModels { get; set; }

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            optionsBuilder.UseMySql("server=localhost;uid=root;pwd=123456;database=repro");
        }
    }
}

Then dotnet ef migrations add Init