Refactor package management API

Sets out to solve a number of issues with the package management
process. Namely:

- To-be-removed packages that are simply being removed are no longer
  incorrectly labeled "quelpa->elpa", but "removed" instead.
- A backend (elpa vs quelpa) column was added to the package listing
  confirmation when running `doom update`.
- Doom now correctly recognizes that packages installed with a psuedonym
  are installed, and will not endlessly attempt to uninstall and
  reinstall them on every `doom refresh`.
- Packages declared with :built-in will no longer lose their built-in
  marking if said package is not actually present in Emacs' site load
  paths. i.e. if you say it's built in, Doom won't question it.
- package!'s :ignore property is now treated as a form whose evaluated
  result will be used as its value.

core/core-packages.el

@@ -197,9 +197,9 @@ elsewhere."
       (unless (member module module-list)
         (setq module-list (append module-list (list module) nil)
               plist (plist-put plist :modules module-list))))
-    (when (and built-in (locate-library (symbol-name name) nil doom-site-load-path))
+    (when built-in
       (doom-log "Ignoring built-in package '%s'" name)
-      (setq plist (plist-put plist :ignore t)))
+      (setq plist (plist-put plist :ignore built-in)))
     (while plist
       (unless (null (cadr plist))
         (setq old-plist (plist-put old-plist (car plist) (cadr plist))))