Deprecate _update_cron_notify() and remove update.fetch.inc

Problem/Motivation

Once upon a time, update.fetch.inc included all the code that runs while fetching new available update data and related support methods. Over time, folks have OO-ified this into src/UpdateFetcher.php, src/UpdateProcessor.php and others. The once huge update.fetch.inc now contains a single, lonely function, _update_cron_notify().

Proposed resolution

• Move the brains from _update_cron_notify() to Drupal\update\Hook\UpdateCronHooks.
• Move the mail sending code to a new Drupal\update\MailHandler to make it swappable (for the same reasons as #3539178: Extract _user_mail_notify() into a user NotificationHandler).
• Remove update.fetch.inc

Remaining tasks

1. Implement the proposed resolution.
2. Reviews.
3. RTBC.
4. Commit.

User interface changes

N/A

API changes

TBD. Nothing public, since _update_cron_notify() is prefixed with an underscore and therefore officially considered internal.

Data model changes

N/A

Release notes snippet

TBD, probably none.