allow building with aeson >= 2.0

allow building with aeson >= 2.0

In aeson 2.0, Text has been replaced by the Key type and HashMap by the
KeyMap interface. Accomodating this required adding some CPP in order to
still be able to compile with aeson < 2.0. The required changes were:

* Prevent Key from being re-exported by Utilities.Aeson, as it clashes
  with git-annex's own Key type.

* Fix up convertion from String/Text to Key (or Text in aeson 1.*) in a
  couple of places

* Import Data.Aeson.KeyMap instead of Data.HashMap.Strict, as they are
  mostly API-compatible. insertWith needs to be replaced by unionWith,
  however, as KeyMap lacks the former function.