correct click script command prefix resolution

[gmanchon]

the click provided code supporting prefixes introduced in #3 for click powered script commands provides some weird resolutions such as tail here which is not a challengify command

➜  taxi-fare git:(challengify-iterate) cha t --help                             [🐍 pyenv lewagon]
Usage: challengify [OPTIONS] COMMAND [ARGS]...
Try 'challengify --help' for help.

Error: No such command 'tail'.

existing challengify commands :

➜  taxi-fare git:(challengify-iterate) cha --help                               [🐍 pyenv lewagon]
Usage: challengify [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  clean    Clean test challengification files
  gen      Generate default parameters conf file in `~/.challengify.yaml`.
  inject   Inject solutions from external sources.
  iterate  Iterate on ssot to generate solutions.
  run      Generate challenges from provided source files and directories...
  test     Proxy for the challengify run command with --test option