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

vars/pkgs/alacritty.yml

@@ -1,10 +0,0 @@
-# vim: set filetype=yaml.ansible :
----
-alacritty:
-  version: 0.15.1
-  method:
-    default: cargo
-    Darwin: cask
-  cargo:
-    locked: true
-    name: alacritty