Replace and update use of deprecated 'linux_distribution'
    
    This uses distro.id and distro.version instead, and adjusts the
    handling appropriately. The value we get here is only ever used
    for comparison with Commands._disttag, which is set in
    _load_rpmdefines_branch. There is handling there for Fedora
    (tag is always 'fcNN'), various forms of EL (with possible tags
    like 'elN', 'elN_N' or 'elN.next'), OLPC (tag 'olpcNN', I think
    this is obsolete) and ELN (tag 'elnNN', I think). disttags that
    show up in the test suite are 'fc26', 'el6', 'el7', 'el8.next',
    'el10', 'el10_10', 'fc28', 'olpc7' and 'eln104'.
    
    This handles Fedora in the obvious way. It handles RHEL, CentOS
    and Alma flavors of EL, assuming branch names (and hence dist
    tags) with major version only; I don't think we can handle
    minor versions, really, because e.g. RHEL or Alma 9.4 report
    their distro.version as '9.4', but we want the tag to be 'el9'.
    I think the major_minor thing is meant for EPEL 10, so *possibly*
    we could handle it with differing behaviour between EL >= 10
    and EL < 10, but I'm really not enough of an expert to know. At
    least this should be no worse than before. It handles ELN by
    checking for the variant if the id is Fedora (ELN looks a lot
    like Fedora but can be distinguished by variant) then doing the
    counterpart to what the dist tag definition does (reading the
    'eln' build macro to get the digits). The old code did not handle
    ELN.
    
    I don't think it's possible to handle EPEL next; the old code
    did not either. I don't think it's worth handling OLPC.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>