Add determine_rhel_state() and does_branch_exist()
    
    These replace determine_active_y_version and does_divergent_branch_exist()
    
    The return value from determine_rhel_state checks for both Product Pages
    state and the presence of the prior release branch to return a complete
    set of information about how the current cXs branch will behave.
    
    This patch updates the cli.request_current_state() and cli._build()
    functions to use these two new utility features. It also refactors those
    two functions to reuse code better.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>