extract_if_needed now extracts relative to the source file's parent
directory instead of /mnt/converter/tmp/in/. This correctly handles
files in tmp/{guid}/in/ paths.
Archives like Debian_13_VMG.7z contain nested single-child dirs:
Debian_13_VMG_LinuxVMImages.COM/Debian_13_VMG_LinuxVMImages.COM/
_flatten_nested() collapses these chains until files sit directly
in the parent, eliminating the double-nesting issue.
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
