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This is because we can't be sure which arguments require a json file
and which don't, so the `else' statement will always be difficult to
check for correctness.

Use $_comp_command1 for command list generation instead of 'composer':

  `$_comp_command1` contains the command written in the command line.
  This allows us to have a different composer name

This plugin currently doesn't autocomplete composer commands if there is no composer.json in the current dir. However, the commands create-project, init, search, selfupdate and show are still useful without a composer.json!