Avoid duplicates when adding package dirs to load-path

Avoid duplicates when adding package dirs to load-path

Do not merge to master, we're going to delete this code there.

* lisp/emacs-lisp/package.el (package-activate-1): Check if the path
we're about to add is already in 'load-path', since package autoload
files have been updating 'load-path' for a decade.