refactor: generalize fd/ripgrep vars & options

I intend to eventually replace projectile with project.el, so these
doom-projectile-* variables need to be generalized, starting with the
fd/ripgrep executable paths.

ALong with that, this refactors Doom's projectile-generic-command to
lean more on built-in fd support in projectile, where possible (fewer
wheels reinvented).

Ref: doomemacs/core#1

modules/emacs/dired/config.el

@@ -211,7 +211,7 @@ we have to clean it up ourselves."
 
 
 (use-package! fd-dired
-  :when doom-projectile-fd-binary
+  :when doom-fd-executable
   :defer t
   :init
   (global-set-key [remap find-dired] #'fd-dired)