Fix handling of SRPMs in Content Generator builds when SRPM name and main package name are different.
    
    Current code presumes SRPM name always matches the RPM name built out of this SRPM
    and only includes it together with the main package in this case. This is wrong
    assumption, because usually there are multiple binary RPMs built from single SRPM.
    
    This commit fixes that by including the SRPM NEVRA in `non_devel_source_rpms`
    no matter what RPM name is.
    
    The test RPMs are reworked in this commit to match the reality better - especially
    the relations between SRPM and RPMs. The case with different SRPM name and RPM
    name is also included in the reworked test - dhcp-libs binary RPM built from
    dhcp SRPM.