Changes

improve rootfstype detection

I just noticed this MR from Shih-Yuan Lee being closed:

  https://salsa.debian.org/installer-team/partman-swapfile/-/merge_requests/1

and inspired by the comment from Samuel Thibault I came up with this
(untested) commit, and only then noticed that partman-swapfile is
effectively dead, so rather than repopen the MR I'll commit this, just
in case anyone decides to revive it at some point.