Leverage loaddefs for migrating ERC modules

Leverage loaddefs for migrating ERC modules

* lisp/erc/erc-common.el (erc--features-to-modules,
erc--modules-to-features, erc--module-name-migrations): Remove unused
internal functions.
(erc--normalize-module-symbol): Make aware of new migration scheme
based on symbol properties.
* lisp/erc/erc-page.el: Add autoload cookie for module migration.
* lisp/erc/erc-pcomplete.el: Add autoload cookies for module
migration.
* lisp/erc/erc-services.el: Add autoload cookie for module migration.
* lisp/erc/erc-sound.el: Add autoload cookie for module migration.
* lisp/erc/erc-stamp.el: Add autoload cookie for module migration.
* lisp/erc/erc.el (erc-modules): Reorder default value, sorted by
`string<' so that Customize does not consider the value to have been
edited.  Remove non-existent module `hecomplete' from lineup and swap
a couple more to maintain sorted order.  Change `:initialize' function
to tag all symbols for built-in modules with an `erc--module'
property.  In the `:set' function, ensure third-party modules appear
after the sorted and normalized built-ins, but in user-defined order.
Do this to prevent all modules, built-ins included, from ending up as
populated form fields for the "other" checkbox in the Customize
interface.
(erc--find-mode): Add helper function for `erc--update-modules'.
(erc--update-modules): Always resolve module names and only
conditionally attempt to require corresponding features.
* test/lisp/erc/erc-tests.el (erc-tests--modules): Add manifest for
asserting built-in modules and features.  This is easier to verify
visually than looking at the custom-type set for `erc-modules'.
(erc-modules--initialize): New test.
(erc-modules--internal-property): Add test.
(erc--normalize-module-symbol): New test.
(erc--find-mode): New test.
(erc--update-modules) Adapt to new paradigm and make more
comprehensive.  (Bug#60954.)