Improve doom doctor

+ Bump Emacs version check to 25.3
+ Fix doctor reporting missing packages that are user-disabled
+ Add Doom core checks for over-sized cache files (a possible cause of
  freezes/hangs)
+ Emit a backtrace from module doctor script errors
+ Fix doom doctor not respecting DEBUG envvar

core/doctor.el

@@ -0,0 +1,29 @@
+;;; core/doctor.el -*- lexical-binding: t; -*-
+
+(defun file-size (file &optional dir)
+  (setq file (expand-file-name file dir))
+  (when (file-exists-p file)
+    (/ (nth 7 (file-attributes file))
+       1024.0)))
+
+;; Check for oversized problem files in cache that may cause unusual/tremendous
+;; delays or freezing. This shouldn't happen often.
+(dolist (file (list "savehist"
+                    "projectile.cache"))
+  (let* ((path (expand-file-name file doom-core-dir))
+         (size (file-size path)))
+    (when (and (numberp size) (> size 2000))
+      (warn! "%s is too large (%.02fmb). This may cause freezes or odd startup delays"
+             (file-relative-name path doom-core-dir)
+             (/ size 1024))
+      (explain! "Consider deleting it from your system (manually)"))))
+
+(when! (not (executable-find "fd"))
+  (warn! "Couldn't find the `fd' binary; project file searches will be slightly slower"))
+
+(let ((default-directory "~"))
+  (require 'projectile)
+  (when! (cl-find-if #'projectile-file-exists-p projectile-project-root-files-bottom-up)
+    (warn! "Your $HOME is recognized as a project root")
+    (explain! "Doom will disable bottom-up root search, which may reduce the accuracy of project\n"
+              "detection.")))