refactor!: restructure Doom core

BREAKING CHANGE: This restructures Doom's core in an effort to slim it
down and partially mirror architectural changes coming in v3. This is
part 2 of 3 commits (part 1 being 1590434), done to facilitate a change
in part 3 that will introduce a new `doom!` syntax for pulling
third-party module libraries from remote sources (similar to `package!`
statements). I am backporting this from V3 so I can move our modules out
into separate repos sooner than later, so development on modules can
continue separately without interfering with v3's roll out.

Though this is labeled a breaking change, it shouldn't affect most users
except those few tinkering directly with Doom's internals.

Ref: 15904349cf

bin/doom

@@ -297,7 +297,7 @@ SEE ALSO:
 
   (let ((cli-file "cli.el"))
     (defcli-group! "Module commands"
-      (with-doom-context 'modules
+      (with-doom-context 'module
         (dolist (key (doom-module-list))
           (when-let (path (doom-module-locate-path key cli-file))
             (defcli-group! :prefix (if (cdr key) (format "+%s" (cdr key)))