Refactor Project API to reflect changes upstream

projectile-project-root no longer returns `default-directory` if not in
a project (it returns nil). As such, doom-project-* functions (and their
uses) have been refactored.

+ doom-project-p & doom-project-root are aliases for
  projectile-project-p & projectile-project-root.
+ doom-project-{p,root,name,expand} now has a DIR argument (for
  consistency, since projectile-project-name and
  projectile-project-expand do not).
+ The nocache parameter is no longer necessary, as projectile's caching
  behavior is now more sane.
+ Removed some projectile advice/hacks that are no longer necessary.
+ Updated unit tests

core/autoload/buffers.el

@@ -65,7 +65,7 @@ scratch buffer. See `doom-fallback-buffer-name' to change this."
 
 If no project is active, return all buffers."
   (let ((buffers (doom-buffer-list)))
-    (if-let* ((project-root (if (doom-project-p) (doom-project-root))))
+    (if-let* ((project-root (doom-project-root)))
         (cl-loop for buf in buffers
                  if (projectile-project-buffer-p buf project-root)
                  collect buf)