Handle removing system imports

[keith]

Right now only your own imports are correctly handled. System imports bring a new set of issues. Here are a few I hit while working on this:

• You have to handle @objc (and other attributes) requiring a Foundation import
• extensions on TimeInterval don't find definition because definition is NSTimeInterval. I think the tool would need to understand apinotes to handle this case.
• Some things come from places you might not expect, and that can be ok. SwiftLint provides a way to map imports. When we were using that we used this config:

unused_import:
  require_explicit_imports: true
  always_keep_imports:
    - AVFoundation # Never translate to AVFAudio, which is only available in 14.5+
  allowed_transitive_imports:
    - module: AVFoundation
      allowed_transitive_imports:
        - AVFAudio
    - module: Foundation
      allowed_transitive_imports:
        - CoreFoundation
        - Darwin
        - ObjectiveC
    - module: CoreMedia
      allowed_transitive_imports:
        - _SwiftCoreMediaOverlayShims
    - module: GoogleMaps
      allowed_transitive_imports:
        - GoogleMapsBase
    - module: GoogleNavigation
      allowed_transitive_imports:
        - GoogleMaps
    - module: UIKit
      allowed_transitive_imports:
        - CoreFoundation
        - CoreGraphics
        - CoreText
        - Darwin
        - ObjectiveC
        - QuartzCore

But this can depend on how strict you want to be. Besides AVFoundation / AVFAudio which you can only fix if you have a high deployment target.

• Some things you might want to allow shadowing of, like GoogleMapsBase vs GoogleMaps in the example config above, where that framework is distributed as 3 separate frameworks to avoid GitHub's file size limit, but realistically you likely want users to only ever import GoogleMaps, so I think this being user configurable is required.
• There are some other cases I haven't debugged enough that cause some issues as well, but this is a sense of the work needed