composer: Fix bin directory when Composer is not available (#6240)

* 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

plugins/composer/composer.plugin.zsh

@@ -51,5 +51,10 @@ alias cgrm='composer global remove'
 # install composer in the current directory
 alias cget='curl -s https://getcomposer.org/installer | php'
 
-# Add Composer's global binaries to PATH
-export PATH=$PATH:$(composer global config bin-dir --absolute 2>/dev/null)
+# Add Composer's global binaries to PATH, using Composer if available.
+if (( $+commands[composer] )); then
+  export PATH=$PATH:$(composer global config bin-dir --absolute 2>/dev/null)
+else
+  [ -d $HOME/.composer/vendor/bin ] && export PATH=$PATH:$HOME/.composer/vendor/bin
+  [ -d $HOME/.config/composer/vendor/bin ] && export PATH=$PATH:$HOME/.config/composer/vendor/bin
+fi