Clarify purpose of module aliases in ERC

Clarify purpose of module aliases in ERC

* doc/misc/erc.texi: Mention that aliases should not be defined for
new modules.
* lisp/erc/erc-common.el (define-erc-module): Refactor slightly for
readability.
(erc-with-all-buffers-of-server): Redo doc string.
* lisp/erc/erc-pcomplete.el: Declare `completion' module's feature and
group as being `erc-pcomplete'.
* test/lisp/erc/erc-tests.el (erc--find-group--real): Assert group
lookup works for "normalized" module name `completion' of
`erc-pcomplete-mode'.