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/cli/env.el

@@ -98,7 +98,7 @@ default, on Linux, this is '$SHELL -ic /usr/bin/env'. Variables in
           "#\n"
           "# It is NOT safe to edit this file. Changes will be overwritten next time that\n"
           "# `doom env refresh` is executed. Alternatively, create your own env file and\n"
-          "# load it with `(doom-load-env-vars FILE)`.\n"
+          "# load it with `(doom-load-envvars-file FILE)`.\n"
           "#\n"
           "# To auto-regenerate this file when `doom reload` is run, use `doom env auto' or\n"
           "# set DOOMENV=1 in your shell environment/config.\n"