Archives like 64bit.7z that already contain a 64bit/ top-level dir
were extracted into STAGING_IN/64bit/, creating 64bit/64bit/ nesting.
Now extract into STAGING_IN root, then:
- If the archive produced a single dir matching the stem, use it
- Otherwise, collect scattered files into a stem-named subdirectory
Removes the need for nested path: 64bit/Debian.vmdk instead of
64bit/64bit/Debian.vmdk