Deploying a packaged stack with an unchanged function fails

[aidansteele]

If you run stackit up twice on a template that contains a serverless function and the code doesn't change in between runs, a failure emerges - when it should be a no-op. Output:

[10:12:16] test-stack-packaged - UPDATE_IN_PROGRESS - User Initiated
[10:12:20] Function - UPDATE_IN_PROGRESS
[10:12:21] Function - UPDATE_COMPLETE
[10:12:23] FunctionVersion7b4af - CREATE_IN_PROGRESS
[10:12:23] FunctionVersion7b4af - CREATE_FAILED - A version for this Lambda function exists ( 1 ). Modify the function to create a new version.
[10:12:24] test-stack-packaged - UPDATE_ROLLBACK_IN_PROGRESS - The following resource(s) failed to create: [FunctionVersion7b4af85c00].
[10:12:40] Function - UPDATE_IN_PROGRESS
[10:12:40] Function - UPDATE_COMPLETE
[10:12:42] test-stack-packaged - UPDATE_ROLLBACK_COMPLETE_CLEANUP_IN_PROGRESS
[10:12:44] FunctionVersion7b4af - DELETE_SKIPPED
[10:12:44] test-stack-packaged - UPDATE_ROLLBACK_COMPLETE

This (and avoiding unnecessary uploads) is likely why the AWS CLI uses the MD5 of the zip archive as the basename in the S3 key. We should do that (see #30)