For example you might have custom source tooling that does something
with types conforming to a procotol even if that type is never
explicitly referenced by other code.
This PR adds a new `unused_declaration` analyzer rule to lint for unused declarations.
By default, detects unused `fileprivate`, `private` and `internal` declarations.
Configure the rule with `include_public_and_open: true` to also detect unused `public` and `open` declarations.
Completely remove the `unused_private_declaration` rule.
This is built on the work enabling collecting rule infrastructure in https://github.com/realm/SwiftLint/pull/2714.
Danny Mösch