Remove dependency on private MacroTransformation API in MacroTransformationContext

Found using http://nemo.sonarsource.org/drilldown/measures/207147?metric=package_cycles&rids[]=215557

MacroTxContext should use Tx and not MacroTx which is an internal implementation.

The change will break an API (Macro writers who have macros using MacroTransformationContext.getMacroTransformation() will be broken and will need to modify their code to use MacroTransformationContext.getTransformation()).