shell: Install emacs-nox iff emacs-gtk is not installed

shell/defaults/main.yml

@@ -2,7 +2,6 @@
 
 shell_software:
   - bash-completion
-  - emacs-nox
   - lftp
   - mercurial
   - pwgen

shell/tasks/shell.yml

@@ -49,6 +49,15 @@
     - packages
     - shell
 
+- name: check installed software
+  package_facts:
+    manager: auto
+
+- name: install emacs-nox iff emacs-gtk is not installed
+  apt:
+    name: emacs-nox
+  when: "'emacs-gtk' not in ansible_facts.packages"
+
 - name: ensure installation of variable additional software
   apt:
     name: "{{ shell_software }}"