[frontend] separate version of the copr-frontend-flavor provide
    
    The version should be changed only if some new customizable file
    is added or if there's some incompatible change (not for every
    major release bump).  It means that the dependent packages don't
    have to be re-built so often.
    
    Also rename the "exported" %copr_frontend_flavor_guard macro into
    %copr_frontend_flavor_pkg, and also make sure that any *-flavor
    package requires 'copr-frontend' package.