Spoof channel users in erc-button--phantom-users-mode

Spoof channel users in erc-button--phantom-users-mode

* lisp/erc/erc-backend.el (erc--cmem-from-nick-function): Update
forward declaration.
(erc-server-PRIVMSG): Use new name for `erc--user-from-nick-function',
now `erc--cmem-from-nick-function'.
* lisp/erc/erc-button.el (erc-button--phantom-users,
erc-button--phantom-cmems): Rename former to latter.
(erc-button--fallback-user-function,
erc-button--fallback-cmem-function): Rename former to latter.
(erc--phantom-channel-user, erc--phantom-server-user): New superficial
`cl-struct' definitions "subclassing" `erc-channel-user' and
`erc-server-user'.  Note that these symbols lack an `erc-button'
prefix.
(erc-button--add-phantom-speaker): Look for channel member instead of
server user, creating one if necessary.  Return a made-up
`erc-channel-user' along with a fake `erc-server-user'.
(erc-button--get-phantom-user, erc-button--get-phantom-cmem): Rename
former to latter.
(erc-button--phantom-users-mode, erc-button--phantom-users-enable,
erc-button--phantom-users-disable): Use updated "cmem" names for
function-valued interface variables and their implementing functions.
Remove obsolete comment.
(erc-button-add-nickname-buttons): Attempt to query fallback function
for channel member instead of server user.
* lisp/erc/erc.el (erc--user-from-nick-function,
erc--cmem-from-nick-function): Rename former to latter.
(erc--examine-nick, erc--cmem-get-existing): Rename former to
latter.  (Bug#60933)