Fix wrong inclusion of non-multilib packages in final MMD.
The original Pungi code, on which MBS code is based on, always passed only RPMs
with valid architectures to further decide if their subset should end up in a
final modulemd file.
In MBS, we pass RPMs with all architecture and there was no code to actually filter
out the RPMs which are from architectures which should never end up in a final MMD.
This commit checks that RPMs for completely different architectures will never
be considered to be included in a final MMD.