[5.7] Fix loadMissing() with duplicate relation names

[staudenmeir]

loadMissing() doesn't work with duplicate relationship names:

$posts = Post::with('comments')->get();
$posts->loadMissing('comments.parent.parent');
                                     ^^^^^^

This is caused by the structure of the internal $path array:

// before
array:2 [
  "comments" => "comments"
  "parent" => "parent"
]

// now
array:3 [
  0 => array:1 [
    "comments" => "comments"
  ]
  1 => array:1 [
    "parent" => "parent"
  ]
  2 => array:1 [
    "parent" => "parent"
  ]
]

Fixes #27039.

[phh]

Can confirm that this fixes my issue in #27039