Don't associate ERC modules with undefined groups

Don't associate ERC modules with undefined groups

* lisp/erc/erc-capab.el: Add property crutch to help ERC find module's
Custom group.
* lisp/erc/erc-common.el (erc--find-group): Add new function, a helper
for finding an existing ERC module's Custom group based on
`define-erc-module' params.  Prefer `group-documentation' as a
sentinel over symbol properties owned by Customize because they might
not be present if the group isn't yet associated with any custom
variables.
(define-erc-module): Set `:group' keyword value more accurately,
falling back to `erc' when no associated group has been defined.
* test/lisp/erc/erc-tests.el (erc--find-group, erc--find-group--real):
New tests.
(define-erc-module--global, define-erc-module--local): Expect the
`:group' keyword to be the unevaluated `erc--find-group'
form.  (Bug#60935.)