确定资源是否为外部资源。
A foreign Asset is a file produced externally to Unity that's imported into a Project (for example, a .png Texture file is a foreign Asset).
Upon importing a foreign Asset, Unity will produce a serialized representation of the data in the Project's Library folder. This is done when the Asset is first added to the Project and when the foreign Asset is updated externally to Unity.
另请参阅：AssetDatabase.IsNativeAsset