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/pipx.yml

@@ -0,0 +1,11 @@
+# vim: set filetype=yaml.ansible :
+---
+- name: Install pipx {{ pkg }}
+  become: "{{ ext_become }}"
+  environment:
+    PIPX_HOME: "{{ path.pipx }}"
+    PIPX_BIN_DIR: "{{ path.bin }}"
+  community.general.pipx:
+    executable: "{{ pipx_exec }}"
+    name: "{{ pkg }}"
+    state: latest