Refactor Doom core init process (again)

- Eager-load all core autoloaded libraries if autoloads file isn't
  present.
- Renames functions to be more descriptive of their true purpose:
  - doom-initialize-autoloads -> doom-load-autoloads-file
  - doom-load-env-vars -> doom-load-envvars-file
- Use doom-module-p instead of featurep! for backend use (the latter is
  mainly syntax sugar for module use, and evaluates at compile/expansion
  time, which may cause hash-table-p errors early in the startup
  process).
- Reorder plist library to prevent load order race condition with the
  functions using the macros that haven't been defined yet.

core/autoload/packages.el

@@ -1,8 +1,5 @@
 ;;; core/autoload/packages.el -*- lexical-binding: t; -*-
 
-(require 'straight)
-
-
 ;;
 ;;; Package metadata