Decouple keep-place-indicator from global ERC module

Decouple keep-place-indicator from global ERC module

* etc/ERC-NEWS: Let users know that `keep-place-indicator' is a wholly
separate module from `keep-place'.
* lisp/erc/erc-goodies.el (erc-keep-place-indicator-setup): Perform
some housekeeping on `erc-keep-place-mode'.
(erc-keep-place-indicator-mode, erc-keep-place-indicator-enable,
erc-keep-place-indicator-disable): Take precautions to work around the
activation state of global module `keep-place', but no longer depend
on it.
(erc--keep-place-indicator-on-global-module): New function to ensure
`erc-keep-place' runs exactly once on `erc-insert-pre-hook',
regardless of whether module `keep-place' is active.
* test/lisp/erc/erc-goodies-tests.el
(erc-goodies-tests--assert-kp-indicator-on,
erc-goodies-tests--assert-kp-indicator-off,
erc-goodies-tests--kp-indicator-populate,
erc-goodies-tests--keep-place-indicator): New helper functions.
(erc-keep-place-indicator-mode,
erc-keep-place-indicator-mode--no-global): Factor out some common
logic and rename former to latter.
(erc-keep-place-indicator-mode--global): New test.  (Bug#59943)