init: Add validation for module defaults
    
    After cloning the repository with defaults, open each file and try to
    check if there is more than one definition for the same module. It's not
    a problem for the compose process, but consumers of the compose would
    get confused and possibly explode. Better alert people early.
    
    Conceptually this should be part of the test phase, but that would mean
    waiting for the compose to finish before reporting the error. The
    earlier the better.
    
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>