[12.x] Add ability for recursive check relation load

[freebuu]

Hello!

I want to present an improve for HasRelationships::relationLoaded function - check loaded state of child relations in dot notation. It same as dot notation in load or with functions and fully backward compatible.
Before

$model->load('foo.bar')
$model->relationLoaded('foo') //true
$model->relationLoaded('foo.bar') //always false

After:

$model->load('foo.bar')
$model->relationLoaded('foo') //true
$model->relationLoaded('foo.bar') //true
$model->relationLoaded('foo.bar.baz') //false

[freebuu]

Ok my fail, it not work with hasMany and cannot work.