* Fix for Composer's bin when Composer isn't global

When Composer isn't globally installed, the `composer global` call results in an error. This checks to see if Composer is available before making the call. When Composer isn't available, it will just manually set the directories.

* Fix Composer brackets in global bin directory

* composer: Apply feedback from ricpelo

This applies ricpelo's feedback at https://github.com/robbyrussell/oh-my-zsh/pull/6240#pullrequestreview-64253321

* composer: Fix path check syntax

* composer: test with $commands[] syntax