Included missing package.artifact declaration

[alastairmccormack]

Without specifying package.artifact, Serverless continues to use its own package

[philip-ulrich]

This PR helped me debug an issue i was having identical to yours. I think it's better to just add the new

package:
  individually: true

then include the artifact in the function package section - especially with the note that since 1.18 the package individually portion is needed according to the readme.

Sadly though this project seems abandoned so no one will likely every merge either change in. Hope it continues to run without issue and no updates. 😄