How to reproduce:
1. Create a new project
2. Import the user's attached package "testing.unitypackage" (Assets>Import Package>Custom Package...)

Expected results: There are no errors in the Console window
Actual results: There are errors in the Console window

Reproducible with: 2019.4.29f1, 2020.3.17f1, 2021.1.18f1, 2021.2.0b7, 2022.1.0a5

Errors:
Assets\Plugins\Arbor\Internal\Scripts\FlexibleField\FlexibleSceneObjectBase.cs(30,13): error CS0246: The type or namespace name 'FlexibleSceneObjectType' could not be found (are you missing a using directive or an assembly reference?)
Assets\Plugins\Arbor\Internal\Scripts\FlexibleField\FlexibleSceneObjectBase.cs(149,10): error CS0246: The type or namespace name 'FlexibleSceneObjectType' could not be found (are you missing a using directive or an assembly reference?)
Could not create asset from Assets/Plugins/Arbor/Internal/Editor/EditorResources/Textures/node header 6.png: File could not be read