bug#41646: Startup in Windows is very slow when load-path contains many

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> I had searched all 200+ packages in my test env, most of the packages
> use their feature name as the prefix, only 11 packages have
> exceptions.

FWIW, I just looked at the longest common prefix for .el files in
(Non)GNU ELPA packages and the ratio is a bit worse, but still good
enough that it should give good speedups:

    (let ((default-directory "../nongnu/packages/"))
      (mapcar (lambda (dir)
                (when (file-directory-p dir)
                  (let ((completion-regexp-list '("\\`[^.].*\\.el")))
                    (list (intern dir)
                          (file-name-completion
                           "" dir)))))
              (directory-files "."))))

In both GNU ELPA and NonGNU ELPA only about 10% of the packages had
a non-empty common prefix, which means that `load` would be able to
skip about 90% of the package directories.


        Stefan