Build checked-out modules by default again

This was desired by Stephan Preibisch and Curtis Rueden made sure
that this developer re-added the support.

There are nasty warning again, of course, because pom-fiji-submodules
is the parent of none of the submodules, yet they are all children...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>