[unused_declaration] False positive when the conformance to a protocol happens on an extension in a different file

New Issue Checklist

• Updated SwiftLint to the latest version
• I searched for existing GitHub issues

Describe the bug

When running the unused_declaration analyze rule:

• Having a protocol and class conforming to that protocol : https://github.com/acecilia/swift-compilation-database/blob/master/Sources/ModuleB/SomethingProtocol.swift
• Having the implementation of the conformance in a different file: https://github.com/acecilia/swift-compilation-database/blob/master/Sources/ModuleB/Something%2Bconformance.swift

The rule will throw a false positive, detecting the conformance that happens in a different file as unused:

swift-compilation-database/Sources/ModuleB/Something+conformance.swift:2:10: error: Unused Declaration Violation: Declarations should be referenced at least once within all files linted. (unused_declaration)

Environment

• SwiftLint version (run swiftlint version to be sure)? 0.45.0
• Installation method used (Homebrew, CocoaPods, building from source, etc)? mint
• Paste your configuration file:

analyzer_rules:
  - unused_import
  - unused_declaration

unused_declaration:
  include_public_and_open: true

• Are you using nested configurations?
  If so, paste their relative paths and respective contents. No
• Which Xcode version are you using (check xcodebuild -version)? Xcode 12.5.1
• Do you have a sample that shows the issue? Yes. Clone https://github.com/acecilia/swift-compilation-database and then do make go_with_xcodebuild

Start with the unused_declaration analyzer and reproduce the report by cloning swift-compilation-database and running make go_with_xcodebuild. Inspect Sources/ModuleB/SomethingProtocol.swift and Sources/ModuleB/Something+conformance.swift, focusing on how declarations and conformances are considered across files. Done means the cross-file conformance is no longer reported as unused while genuine unused declarations remain detected.