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/scala/config.el

@@ -1,5 +1,12 @@
 ;;; lang/scala/config.el -*- lexical-binding: t; -*-
 
+(after! projectile
+  (add-to-list 'projectile-project-root-files "build.sbt"))
+
+
+;;
+;;; Packages
+
 (after! scala-mode
   (setq scala-indent:align-parameters t
         ;; indent block comments to first asterix, not second