fix: External refs with overrides

[simonbothen-radinn]

The external resolver did not take into account for overrides. E.g take the following example:

$ref: 'base.yaml'
id: my_id
settings:
  $ref: 'base_settings.yaml'
  useCache: true

This failed to resolve on $ref: 'base_settings.yaml'. This is due to to root object passes $Ref.isExternal$Ref(obj) and therefore not considering the other child objects. We should still crawl the tree even though the root has a $ref.

[jonluca]

Included in #321