Clarify output path logic for unpack in vignettes

pkglite follows a simple rule when unpacking packages:

• It will create directory(s) under the output directory named after the parsed package name(s) from each DESCRIPTION file of the package(s), and put the content of each package under them.
• This design implies that the simplest way to avoid unnecessary confusions is to keep package names and folder names an identical mapping.
• Users can adjust the directory names and hierarchy in userland easily, but that would belong to individual business logic.