Merge duplicated imports in finally-generated Dangerfile

[417-72KI]

Dividing files using // fileImport: is a very useful feature on Danger-Swift.

However, importing module in extracted file may causes an error.

Example

Dangerfile.swift

import Danger

let danger = Danger()

do {
  // fileImport: DangerExtensions/SomeFeature.swift
  try doSomething(danger, file: "path/to/file")
} catch {
  logger.error(error)
  danger.fail(error.localizedDescription)
}

DangerExtensions/SomeFeature.swift

import Danger // - error: declaration is only valid at file scope

func doSomething(_ danger: DangerDSL, filePath: String) throws {
  // ...
}

This PR makes it to merge duplicated module imports and declare at the top on generated _tmp_dangerfile.swift.