unused_import not taking into account @_exported declarations.

[rz-robsn]

New Issue Checklist

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

Describe the bug

Running the analyzer with the unused_import rule enabled sometimes wrongfully deletes an import if the imported Module contains a @_exported declaration.

Consider the following example :

/// Module A
@_exported import B

/// Module B
public class MyBClass {}

/// Module C
import A

let bClass = MyBClass()

Running the analyzer would currently remove the import A declaration from Module C, leading to a compile error.

Environment

• SwiftLint version (run swiftlint version to be sure)?
  0.35.0

• Installation method used (Homebrew, CocoaPods, building from source, etc)?
  Downloaded binary from the github release page

• Paste your configuration file:

whitelist_rules:
  - closure_spacing
  - colon
  - empty_enum_arguments
  - fatal_error_message
  - force_cast
  - force_try
  - force_unwrapping
  - implicitly_unwrapped_optional
  - legacy_cggeometry_functions
  - legacy_constant
  - legacy_constructor
  - legacy_nsgeometry_functions
  - operator_usage_whitespace
  - redundant_string_enum_value
  - redundant_void_return
  - return_arrow_whitespace
  - trailing_newline
  - type_name
  - unused_closure_parameter
  - unused_optional_binding
  - vertical_whitespace
  - void_return
  - custom_rules

analyzer_rules:
  - unused_import

excluded:
  - Carthage
  - Pods

colon:
  apply_to_dictionaries: false

indentation: 2

custom_rules:
  no_objcMembers:
    name: "@objcMembers"
    regex: "@objcMembers"
    message: "Explicitly use @objc on each member you want to expose to Objective-C"
    severity: error

• Are you using nested configurations?
  No

• Which Xcode version are you using (check xcode-select -p)?
  10.3

[jpsim]

Fixed in #2993.