[5.2] Add loadOnly() method to only load specified relations

[JayBizzle]

Following on from #14031

Say we have three Models User, UserData and UserComments.

User has a userData relationship that we always want to eager load, so we define that in the $with attribute. It also has a userComments relationship that we eager load as required.

Now, consider this...

$users = User::without('userData')->get();

$users->load('userComments');

This will eager load the specified userComments relationship AND also load the userData relationship. This is perhaps a little confusing, because that is not what we wanted to do.

Not sure if this is expected behaviour, or an oversight in the behaviour of the load() method...current behaviour seems counter intuitive.

To maintain backwards compatibility, I suggest a new method called loadOnly(), which will actually load ONLY the specified relationships.

[taylorotwell]

Going to hold off on this.