refactor(lib): tidy doom-log output

This commit reduces the debug log noise, makes it easier to
read/parse/search, and soft-introduces a convention for doom-log
messages, where they are prefixed with a unique identifier loosely named
after it's running context or calling function.

I haven't enforced it everywhere doom-log is used yet, but this is a
start.

lisp/cli/autoloads.el

@@ -212,7 +212,7 @@ non-nil, treat FILES as pre-generated autoload files instead."
     (dolist (file files (nreverse (delq nil autoloads)))
       (when (and (not (seq-find (doom-rpartial #'string-match-p file) exclude))
                  (file-readable-p file))
-        (doom-log "Scanning %s" file)
+        (doom-log "loaddefs:scan: %s" file)
         (setq file (file-truename file))
         (with-temp-buffer
           (if literal