Reduce list of root files projectile searches for

And define them on a per-module basis. This reduces the number of file
checks it performs, particularly for non-project files.

Might help #1317

modules/lang/elixir/config.el

@@ -1,5 +1,12 @@
 ;;; lang/elixir/config.el -*- lexical-binding: t; -*-
 
+(after! projectile
+  (add-to-list 'projectile-project-root-files "mix.exs"))
+
+
+;;
+;;; Packages
+
 (use-package! elixir-mode
   :defer t
   :init