search plugin should become external for 8.1

[dgw]

The search plugin is kind of hacky, and often breaks for reasons outside of our control (e.g. IP reputation filtering or other anti-abuse measures). It would be great to replace it with an optional package that has more robust functionality, perhaps using duckduckgo-search (actively maintained as of writing; last update less than a week ago).

Once a suggested replacement is published, the built-in search plugin can be slated for removal.

[dgw]

sopel-search has been published and installed on our official Sopel instance; #2613 should handle everything we need related to this, once some type-checking shenanigans are handled by #2614. (Man, you leave some code alone for a month or two and BOOM, new errors without touching anything!)