Move resolving git reference to config validation
    
    Instead of multiple places handling the same thing duplicating the
    logic, it's better to do it once upfront. This allows easy caching of
    the results.
    
    Additional advantage of this approach is that the config dump will
    include resolved URLs. The original reference will still be available in
    the copy of the original config.
    
    JIRA: COMPOSE-3065
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>