Fix libmodulemd usage for changes in libmodulemd-1.5
    
    From libmodulemd 1.4 to 1.5, many mmd.props.X were changed to copy
    X rather than returning a pointer to X - this means that something
    like mmd.props.rpm_api.add(pkg) no longer works as expected. Change
    these usages to mmd.props.peek_rpm_api().add(pkg).