Sort and dedupe when loading modules in erc-open

* doc/misc/erc.texi: Add new subheading "Module Loading" under the
"Modules" chapter.
* lisp/erc/erc.el (erc--sort-modules): New utility function to sort
and dedupe modules.
(erc-modules): In `custom-set' function, factor out collation into
separate utility `erc--sort-modules'.
(erc-update-modules): Call `erc--update-modules' with an argument, the
current value of `erc-modules'.
(erc--aberrant-modules): New variable, a list of symbols whose modules
ERC suspects of being incorrectly defined.
(erc--warn-about-aberrant-modules): New function to print an error
message and emit a warning prior to connecting when
`erc--aberrant-modules' is non-nil.
(erc--find-mode): Make heuristic more robust by always checking for a
mode activation command rather than just a state variable.  This fixes
a compatibility bug, new in 5.6, affecting third-party modules that
autoload module definitions instead of their corresponding
mode-activation commands.
(erc--update-modules): Add new positional argument `modules'.
(erc--setup-buffer-hook): Add new default member,
`erc--warn-about-aberrant-modules'.
(erc-open): Pass sorted `erc-modules' to `erc--update-modules'.
* test/lisp/erc/erc-tests.el (erc--sort-modules): New test.
(erc-tests--update-modules): New fixture.
(erc--update-modules): Remove and rework as three separate tests
dedicated to specific contexts.  The existing one had poor coverage
and was difficult, if not impossible, to follow.
(erc--update-modules/unknown, erc--update-modules/local,
erc--update-modules/realistic): New tests.  (Bug#57955)

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