This commit basically grepped the code base for all uses of Dir.mktmpdir
and Tempfile.new/open and ensures the value is unique within the
code base and also prefixed with `vagrant-`.
Previously, most invocations of these commands simply used "vagrant",
thus making them indistinguishable when trying to identify leaks.
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
Seth Vargo