RPM Support

Steps to reproduce.

1. Create and sync a modular repo with one of the packages in a module present in a different repo. I used https://dhcp-2-152.vms.sat.rdu2.redhat.com/pub/sajha/zoo5/ as the remote for the modular repo. Note that the kangaroo module has two rpms in the module.yaml artifact list. One of them camel is not present in this repo.
2. Create and sync a 2nd repository providing the missing package. I used https://jlsherrill.fedorapeople.org/fake-repos/needed-errata/ which provides the camel package.
3. Re-sync the modular repo and check if the module kangaroo's package list provides href for the newly synced camel package.
4. I tried syncing the 2nd repo before the 1st repo to see if that helped but didn't seem to.

PS: If I add the camel package to the modular repo itself and sync, I see the href added to the packages list. So it seems packages outside the repo are not getting added to the package list.

I am on master branches of pulpcore and pulp_rpm.