WIP: element: Add BST_GRANULAR_STAGE Element flag

This commit introduces BST_GRANULAR_STAGE as a compatibility flag,
so Elements can assert that calling stage_dependency_artifacts,
integrate_dependencies and post_integration_staging
is equivalent to calling stage.

This will be needed to implement staging multiple elements,
since the integration commands for all elements need to be called
after all dependencies are staged.

NOTE: This is WIP because the *Since* markers aren't correct.