Refactor autoloads generator & reduce to one generated file

We no longer need two separate autoloads files, so I merged them and
optimized its generation logic.

Other changes
- Doom will refuse to start up (with a helpful error) if it's in an
  incomplete state. This should hopefully reduce the number of bug
  reports from folks that have done something weird, e.g.
  1. You've changed Emacs versions without running 'doom sync -b'.
  2. You've updated Doom outside of `doom upgrade` and didn't run `doom
     sync -u`.
  3. You've forgotten to run 'doom sync' in the first place!
  4. If a previous 'doom ...' command was aborted midway without running
     'doom sync' afterwards.
- 'doom sync' will emit reminders that you need to reload/restart Emacs
- Autoloads API now uses the `doom-autoloads-` prefix, intead of
  'doom-cli-autoloads-', as will be the new convention in the coming
  rewrite.
- Errors from within the package autoloads should be easier to invoke
  the debugger on.
- `doom-modules` is now stored in your autoloads file. Your module list
  will soon be frozen between calls to 'doom sync' to allow for our new,
  atomic CLI I'm working on. This will also means the `doom!` block
  won't cost anything in interactive sessions.

core/cli/upgrade.el

@@ -21,7 +21,7 @@ following shell commands:
                 (doom-cli-upgrade doom-auto-accept doom-auto-discard))
               (doom-cli-execute "sync")
               (when (doom-cli-packages-update)
-                (doom-cli-reload-package-autoloads)
+                (doom-autoloads-reload)
                 t)))
         (print! (success "Done! Restart Emacs for changes to take effect."))
       (print! "Nothing to do. Doom is up-to-date!"))))