pathutil: use `finddirs_rev_noroot` instead of `parts`
The benefit this brings is very tiny, if it's even there,
since we still didn't get rid of the [parts] computation.
It probably won't be worth it without the subsequent patch
that adds one more use of [finddirs_rev_noroot]
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors