Use 'hash-table-contains-p' in a few places

Use 'hash-table-contains-p' in a few places

This replaces open coded versions of the common idiom
    (not (eq (gethash key table 'missing) 'missing))
with
    (hash-table-contains-p key table)
in files where we can rely on features in Emacs 31.

* lisp/emacs-lisp/map.el (map-contains-key):
* lisp/external-completion.el (external-completion-table):
* lisp/mh-e/mh-utils.el (mh-sub-folders)
(mh-remove-from-sub-folders-cache):
* lisp/net/ange-ftp.el (ange-ftp-hash-entry-exists-p):
* lisp/password-cache.el (password-in-cache-p, password-cache-add):
* lisp/pcmpl-x.el (pcmpl-x-tlmgr-action-options):
* lisp/xdg.el (xdg-mime-apps): Use 'hash-table-contains-p'.

(cherry picked from commit f60fc1287d499e8c93857b1b96e8bd2467b22c8d)