When a sub-module doesn't contain any definitions, we don't generate
code for it, but we do generate the #include.

We generate code only for modules that get visited.
QAPISchema.visit() visits only modules that have definitions.  It can
visit modules multiple times.

Clean this up as follows.  Collect entities in their QAPISchemaModule.
Have QAPISchema.visit() call QAPISchemaModule.visit() for each module.
Have QAPISchemaModule.visit() call .visit_module() for itself, and
QAPISchemaEntity.visit() for each of its entities.  This way, we visit
each module exactly once.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-6-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>