qapi: Proper intermediate representation for modules

Modules are represented only by their names so far.  Introduce class
QAPISchemaModule.  So far, it merely wraps the name.  The next patch
will put it to more interesting use.

Once again, arrays spice up the patch a bit.  For any other type,
@info points to the definition, which lets us map from @info to
module.  For arrays, there is no definition, and @info points to the
first use instead.  We have to use the element type's module instead,
which is only available after .check().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191120182551.23795-5-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>