Some fixes to funtion requires_included
* Fix argument description for requires_included
* Remove comment for the possible negative stream in module metadata. Each
module build got from MBS is the result of module expansion stream (aka MSE)
from original module build submitted to MBS. Ursa-Major just works with such
kind of module builds rather than the original module build with original
modulemd created in module dist-git repository. Hence, it is ok to remove the
comment from function requires_included.
* Simplify the code a bit.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>