feat(plugins): Plugin "debian": Switch order of "apt" and "aptitude" for detection (#7533)

* Switch order of "apt" and "aptitude" for detection

"apt" is installed by default at Debian (maybe Ubuntu too), while "aptitude" does not seem to be installed by default.
For that, it may be better for most of the users to prefer "aptitude" if installed.

* plugins/debian/README: Change of order mirroded into documentation

plugins/debian/README.md

@@ -10,7 +10,7 @@ plugins=(... debian)
 
 ## Settings
 
-- `$apt_pref`: use apt or aptitude if installed, fallback is apt-get.
+- `$apt_pref`: use aptitude or apt if installed, fallback is apt-get.
 - `$apt_upgr`: use upgrade or safe-upgrade (for aptitude).
 
 Set `$apt_pref` and `$apt_upgr` to whatever command you want (before sourcing Oh My Zsh) to override this behavior.

plugins/debian/debian.plugin.zsh

-# Use apt or aptitude if installed, fallback is apt-get
+# Use aptitude or apt if installed, fallback is apt-get
 # You can just set apt_pref='apt-get' to override it.
 
 if [[ -z $apt_pref || -z $apt_upgr ]]; then
-    if [[ -e $commands[apt] ]]; then
-        apt_pref='apt'
-        apt_upgr='upgrade'
-    elif [[ -e $commands[aptitude] ]]; then
+    if [[ -e $commands[aptitude] ]]; then
         apt_pref='aptitude'
         apt_upgr='safe-upgrade'
+    elif [[ -e $commands[apt] ]]; then
+        apt_pref='apt'
+        apt_upgr='upgrade'
     else
         apt_pref='apt-get'
         apt_upgr='upgrade'