lib/types: Handle submodules for type "either"

So far the "either" type only handled "flat" types, so you couldn't do
something like:

type = either int (submodule {
  options = ...;
});

Not only caused this the submodule's options not being checked but also
not show up in the documentation.

This was something we stumbled on with #13916.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @edolstra