Loaders who extend AbstractAssetJsonLoader learn to avoid load resources twice

 For sample, imagine
  - a folder "A" and a subfolder "B",
  - a loader "A" to scan folder "A",
  - a loader "B" to scan folder "A/B".

 Before, the loader "A" load all resources under folder "A" included resources under "A/B".
 Now, the loader "A" load all resources under folder "A" without resources under "A/B".