element: use BST_GRANULAR_STAGE in prepare_sandbox and assemble

This makes it prefer to call stage_dependency_artifacts,
integrate_dependencies and post_integration_staging in-turn
if BST_GRANULAR_STAGE is set,
so the implementation of Element.stage may be omitted.

The prepare_sandbox implementation looks a little convoluted
because for Scope.BUILD and self.BST_GRANULAR_STAGE
we prefer to run the granular methods,
but we also run most of them for Scope.RUN
except the extra parts of stage() that were post-integration.