Restore module var toggles in ERC's Custom buffers

Restore module var toggles in ERC's Custom buffers

* lisp/erc/erc-common.el (erc--neuter-custom-variable-state): Remove
function.  ERC famously toggles global minor-mode vars during normal
operations, which adds noise to its customization buffers because
`customize-variable-state' always sees an activated module's mode
variable as having "CHANGED".  To suppress this annoyance, a
workaround was employed that used a dishonest `:get' function to
simply return the "saved value," when present.  While this improved
the Customize experience, it also misled users, which likely wasn't
justified.
(erc--make-show-me-widget): Add helper to avoid forward declarations.
(erc--prepare-custom-module-type): Don't deprive users of a working
minor-mode toggle.
(erc--find-feature): New function to guess the feature of a module's
containing library.
(define-erc-module): Remove `:get' keyword.  Specify `:require'
instead, whose value may be nil.  Users who currently have mode vars
in their `custom-file' won't be impacted by this addition because
those `custom-set-variables' entries will still lack a REQUEST list
and hence won't incur a startup penalty.  And new users intent on
using the toggle will hopefully do so with the knowledge they're
opting in to requiring ERC on startup, which is not the case if they
follow the recommended practice of using `erc-modules' instead.
(erc-with-server-buffer): Inline `erc-server-buffer'.
* test/lisp/erc/erc-tests.el (erc-process-input-line): Use helper.
(define-erc-module--global): Change expected expansion.  (Bug#60935)