formatted composer completion and used helper function

completion/available/composer.completion.bash

@@ -1,7 +1,9 @@
 #!/usr/bin/env bash
 
-_composer()
-{
+if _command_exists composer
+then
+    function _composer()
+    {
         local cur script coms opts com
         COMPREPLY=()
         _get_comp_words_by_ref -n : cur words
@@ -128,6 +130,7 @@ _composer()
 
             return 0
         fi
-}
+    }
 
-complete -o default -F _composer composer
+    complete -o default -F _composer composer
+fi