gather: Support dotarch in DNF backend
    
    The documentation claims that dotarch syntax is supported for additional
    packages. For yum backend this seems to be handled automatically, but
    the dnf backend could not interpret this.
    
    This patch checks if a package is specified in the syntax and contains a
    valid architecture. If so, the query will honor the arch.
    
    JIRA: RHELCMP-11728
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>