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/cargo.yml

@@ -0,0 +1,11 @@
+# vim: set filetype=yaml.ansible
+---
+- name: Install cargo {{ pkg }}
+  become: "{{ ext_become }}"
+  environment:
+    RUSTONIG_SYSTEM_LIBONIG: 1
+  community.general.cargo:
+    name: "{{ pkg.name | default(pkg) }}"
+    version: "{{ pkg.vers | default(omit) }}"
+    path: "{{ path.cargo | default(path.prefix) }}"
+    locked: "{{ pkg.locked | default(omit) }}"