.serverless folder is not cleaned up before deployment

[medikoo]

.serverless hosts packaged function artifacts.

Currently the artifacts from previous deployment are not cleared with new one.
This creates two issues:

• Stale, not longer needed files pile up in .serverless
• In corner case scenarios files from previous deployments are used when they should not be. (e.g. if we suddenly try to deploy with package:individually setting which is not supported, then plugin ignores fact that expected artifacts were not created, as it sees ones created in previous deployment, and uses them)