* lisp/filecache.el: Use lexical-binding. Simplify.

* lisp/filecache.el: Use lexical-binding.  Simplify.

Remove redundant :group args, use with-tmep-buffer and let
completion-all-completions do its work.

(file-cache-minibuffer-complete): Skip `completion-hilit-commonality`
since `completion-all-completions' already hilighted its output.
Call our setup function directly rather than via `completion-setup-hook`.
(file-cache-buffer): Remove variable.
(file-cache-buffer-default-regexp): Make it a defvar.
(file-cache--add-from-buffer): Rename from
file-cache-add-from-file-cache-buffer and use current-buffer and
match-string-no-properties.
(file-cache-add-directory-using-find)
(file-cache-add-directory-using-locate): Adjust accordingly and use
with-temp-buffer.