Revert "Remove obsolete workaround in dirserv_thinks_router_is_hs_dir()"

Fixes #15850, part of #15801. Change file is added by this commit. The
original comment in the reverted commit is removed because right now we
*need* a DirPort until #15849 is implemented so no doubt nor confusion there
anymore.

This reverts commit 80bed1ac.

Signed-off-by: David Goulet <dgoulet@ev0ke.net>