The absolute path to the root package/project for which to load the selections file.
Either null (if no selections file exists or parsing encountered an error), or a SelectionsFileLookupResult. Note that the nested SelectionsFile might use an unsupported version (see SelectionsFile documentation).
Loads the selections file (dub.selections.json)
The selections file is only used for the root package / project. However, due to it being a filesystem interaction, it is managed from the PackageManager.