[5.x] Fix linking to addon fields

[edalzell]

In our site builder, we have field sets that link to other fieldset fields:

  -
    handle: image_asset
    field: 'simple::image_asset.image_asset'

When you try to load a blueprint containing this, you get an error :

ErrorException: Undefined array key "type"

/Users/erin/Development/cms/vendor/laravel/framework/src/Illuminate/Foundation/Bootstrap/HandleExceptions.php:256
/Users/erin/Development/cms/src/Fields/FieldTransformer.php:152
/Users/erin/Development/cms/src/Fields/FieldTransformer.php:128

This is because Statamic's Arr helper class is used and it converts :'s to '.' so you get simple..image_asset.image_asset as the key, which isn't found.

[edalzell]

Not sure why the linting failed, it's not code I changed.