[5.6] Fix relationships with global scope columns

[staudenmeir]

BelongsToMany relationships don't respect global scopes that limit the selected columns:

class Role extends Model
{
    public static function boot()
    {
        parent::boot();

        static::addGlobalScope(function ($query) {
            $query->select('roles.id');
        });
    }
}

class User extends Model
{
    public function roles()
    {
        return $this->belongsToMany(Role::class);
    }
}

User::first()->roles;

# expected
select "roles"."id", "role_user"."user_id" as "pivot_user_id", [...]

# actual
select "roles"."id", "roles".*, "role_user"."user_id" as "pivot_user_id", [...]

This issue also affects HasManyThrough relationships.

Fixes #22482.

[KKSzymanowski]

Thank you!