Reference properties of attached entities are not considered as loaded (IsLoaded==false)

[jirikanda]

When I attach a graph of objects in EF Core 3.0, reference properties are not marked as loaded (IsLoaded is false).
DbContext tracks a root entity, it also tracks a referenced entity, so I think it should be marked as loaded. In EF 2.2 they was marked as loaded.

(Low priority for me.)

Steps to reproduce

• Create unit test project
• Add Microsoft.EntityFrameworkCore.InMemory
• Add and run the following unit test

	[TestClass]
	public class IsLoadedPropertyReproSteps
	{
		[TestMethod]
		public void IsLoadedPropertyReproSteps_PropertiesOfAttachedEntityShouldBeMarkedAsLoaded()
		{
			// Arrange
			Car car = new Car
			{
				Id = 5,
				Color = new Color
				{
					Id = 60,
					Name = "Red"
				}
			};

			MyDbContext dbContext = new MyDbContext();

			// Act
			dbContext.Attach(car);

			// Assert
			Assert.AreEqual(dbContext.Entry(car).State, EntityState.Unchanged); // OK, tracked
			Assert.AreEqual(dbContext.Entry(car.Color).State, EntityState.Unchanged); // OK, tracked			

			Assert.IsTrue(dbContext.Entry(car).Navigation(nameof(Car.Color)).IsLoaded); // fails
		}
	}

	public class MyDbContext : DbContext
	{
		protected override void OnModelCreating(ModelBuilder modelBuilder)
		{
			base.OnModelCreating(modelBuilder);
			modelBuilder.Entity<Car>();
			modelBuilder.Entity<Color>();
		}

		protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
		{
			base.OnConfiguring(optionsBuilder);

			optionsBuilder.UseInMemoryDatabase(nameof(MyDbContext));
		}
	}

	public class Car
	{
		public int Id { get; set; }
		public Color Color { get; set; }
	}

	public class Color
	{
		public int Id { get; set; }
		public string Name { get; set; }
	}

Further technical details

EF Core version: 3.0
Database provider: Microsoft.EntityFrameworkCore.InMemory
Target framework: .NET Core 3.0