This commit separates the scratch and output directory creation from the
main package middleware into its own PackageSetupFolders middleware.
Additionally, the validation that ensures an output file does not exist
is moved into a validation function that can be shared across multiple
methods.
This refactor permits a pre-flight check to ensure box packaging would
be successful before actually stopping the VM.
Fixes GH-7351
Björn Brala
Seth Vargo