As a user I would like to add modulemd and modulemd-defaults to the skip list
Pulp rpm repo type now lets you skip the following units when syncing rpm, drpm, srpm, erratum and distribution.
For consistency it might make more sense to add modulemd and modulemd-defaults also.
When the user specifies a modulemd to be skipped, the RPMs for that module should not be synced either. Modulemd-defaults should not be synced either.