Rework mutual dependency between erc and erc-backend

Rework mutual dependency between erc and erc-backend

* lisp/erc/erc.el: Declare needed variables exported by erc-backend.el
as special near the top of the file, and only require `erc-backend'
after providing `erc' as a feature at the very end.

* lisp/erc/erc-backend.el: Don't preemptively provide `erc-backend'.

* test/lisp/erc/erc-tests.el (erc--meta--backend-dependencies): Add
utility test to scrape for unused vars that may accumulate over time.
(Bug#54825)