Rather than macos/Flutter containing a mixture of files that should and
shouldn't be checked in, create clear locations for:
- Files that are "owned" by Flutter, but should be checked in
  (Flutter/). This will contain files like the top-level Flutter
  xcconfigs, generated plugin registrants, etc.
- Files that are generated by Flutter on the fly, and should not be
  checked in (Flutter/ephemeral/). This will contain Flutter SDK caches,
  the generated xcconfig, etc.

Also adds Flutter-owned Debug and Release xcconfig variants, in
preparation for PodSpec tooling.