Consolidate messages about modules merged into core if more than one module detected.

@quicksketch brought this up in https://github.com/backdrop/backdrop-issues/issues/478#issuecomment-1394...

At this point is it likely or worth worrying about a site that has had two or more of these modules installed, and so would be getting more than one of these (rather long) messages?

These messages are copy-paste with some adjustments (module name, change notification and link etc.):

Backdrop core now provides a bundled [module_name] module. A different copy of [module_name] module has been located at [module/path]. Remove this module from your installation to use new [module_name] module. For more information see the [module_change_notification_link].

I propose we consolidate these messages when more than one such merged-in-core module is detected to something along the lines of:

Some of the contributed modules used in this Backdrop installation are now bundled with core. Remove these modules from your installation to use the new ones. For more information, see the respective change notification for each module listed below: - [module1_name] at [module1_path] [module1_change_notification_link] - [module2_name] at [module2_path] [module2_change_notification_link] - ...