Purge excess from projectile's cache

Bloated projectile caches can cause freezing and delays. This is
especially so if you accidentally index $HOME or /, which will bloat
them into the hundreds of MBs.

+ Adds purging of blacklisted, over-sized and non-projects from
  projectile's cache when Emacs is killed.
+ Projectile's project file cache will now expire after a week.
+ Corrects file paths in core/doctor's filesize checks.

core/doctor.el

@@ -10,7 +10,7 @@
 ;; delays or freezing. This shouldn't happen often.
 (dolist (file (list "savehist"
                     "projectile.cache"))
-  (let* ((path (expand-file-name file doom-core-dir))
+  (let* ((path (expand-file-name file doom-cache-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"