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.
The MIT license doesn't require that all files be prepended with this
licensing or copyright information. Realm confirmed that they're ok with this
change. This will enable some companies to contribute to SwiftLint and the
date & authorship information will remain accessible via git source control.
JP Simard