Put most erc autoloads in a dedicated file erc-loaddefs.el

Put most erc autoloads in a dedicated file erc-loaddefs.el

These are features that are only useful after erc.el is loaded.
* lisp/erc/erc.el (top-level): Load erc-loaddefs.
* lisp/erc/erc-autoaway.el, lisp/erc/erc-button.el:
* lisp/erc/erc-capab.el, lisp/erc/erc-compat.el:
* lisp/erc/erc-dcc.el, lisp/erc/erc-desktop-notifications.el:
* lisp/erc/erc-ezbounce.el, lisp/erc/erc-fill.el:
* lisp/erc/erc-identd.el, lisp/erc/erc-imenu.el:
* lisp/erc/erc-join.el, lisp/erc/erc-list.el, lisp/erc/erc-log.el:
* lisp/erc/erc-match.el, lisp/erc/erc-menu.el:
* lisp/erc/erc-netsplit.el, lisp/erc/erc-notify.el:
* lisp/erc/erc-page.el, lisp/erc/erc-pcomplete.el:
* lisp/erc/erc-replace.el, lisp/erc/erc-ring.el:
* lisp/erc/erc-services.el, lisp/erc/erc-sound.el:
* lisp/erc/erc-speedbar.el, lisp/erc/erc-spelling.el:
* lisp/erc/erc-stamp.el, lisp/erc/erc-track.el:
* lisp/erc/erc-truncate.el, lisp/erc/erc-xdcc.el:
Set generated-autoload-file to "erc-loaddefs.el".