Download schema relative to root url

[Anteo95]

This PR fixes downloading schema when "rootURL" is provided.

Currently, even though rootURL can be provided as parameter to function

  public static func fetch(
    configuration: ApolloSchemaDownloadConfiguration,
    withRootURL rootURL: URL? = nil
  )

it is actually not used internally on all places where it should be. This causes schema to be downloaded into incorrect folder.

[apollo-cla]

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[calvincestari]

Codegen CLI Integration Tests are failing because of a CocoaPods error with forks - merging anyways..