Fix mapping the distribution package name in the MANIFEST to
what is available on-disk to fix installing distribution sets
that are available locally, but do not directly match what is
shown on the distribution selection menu.

Instead of doing any further string manipulation to create the
menu, just use the distribution set name without the '.txz'
suffix.

At this point, the fourth column in the MANIFEST is likely not
needed anymore, but it won't be removed yet until I am certain.

Sponsored by: The FreeBSD Foundation