hybrid: Optimize getting lookaside packages
    
    The original code ended up downloading all repodata from the lookaside
    repo. This could cause a lot of memory to be used.
    
    The new code only downloads the repomd.xml and then primary record,
    which is sufficient to obtain all needed information. A lot less memory
    is used and the code is also significantly faster.
    
    Here are some alternative ways of getting a list of packages from the
    lookaside repo and reasons why they did not work:
    
     * dnf repoquery - this doesn't include modular packages unless the
       stream is default
     * dnf reposync - requires `--urls` option to only print the names,
       which is not available on RHEL 7
    
    JIRA: RHELCMP-4761
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>