Use AssetType for finding valid prefix subfolders

Also simplifies AssetType enum,
makes it an IntEnum, and adds unique check
5 jobs for open-editor-at-obj-186 in 6 minutes and 23 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_36 #159528
docker

00:05:12

passed build_macosx_36 #159529
macosx

00:06:23

failed build_linux_36 #159453
docker

00:02:37

failed build_linux_36 #159316
docker

00:02:25

passed build_macosx_36 #159317
macosx

00:17:15