FEATURE: subcommands of a callable command are not allowed. e.g.:

if the following exist as callable commands:
cmd1_cmd2
cmd1_cmd2_cmd3

then cmd1_cmd2 is resolved, but cmd1_cmd2_cmd3 is not

UNFIXED: There is still a case of syntax-error or --help call where the best
possible help message is not resolved:

cmd1_cmd2_cmd3 where cmd1_cmd2 is not a command