Modifying default variables and config

Renamed variables for defaults:
- flatpak_method -> default_flatpak_method: system
- defaults.paths.prefix -> default_install_prefix: /usr/local
New variables:
- default_install_method: package

tasks/helpers/addpkg.yml

@@ -0,0 +1,5 @@
+# vim: set filetype=yaml.ansible :
+---
+- name: "Add include task for {{ pkg }}"
+  ansible.builtin.include_tasks:
+    file: "pkgs/{{ pkg }}.yml"