When reusing a source image (Copy flow), the archive was already
extracted in /mnt/converter/in/. The extraction step failed with
'Destination path already exists' from 7z.
Now extract_if_needed checks if the expected output directory already
contains disk images, and returns it directly without re-extracting.
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
