Conditionally packaging a buildpack or an extension

[pacostas]

Summary

This PR changes the package.sh script to conditionally package a buildpack or an extension according to whether is an extension or a buildpack.

The way this is implemented is to check whether the extension file exists on the directory, otherwise, it fallbacks to the default behavior of packaging a buildpack.

Test results:

• node-engine: https://github.com/pacostas/node-engine/pull/48
• ubi-nodejs-extension: https://github.com/pacostas/ubi-nodejs-extension/pull/67

Use Cases

Checklist

• [x] I have viewed, signed, and submitted the Contributor License Agreement.
• [ ] I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [x] I have added an integration test, if necessary.
• [x] I have reviewed the styleguide for guidance on my code quality.
• [x] I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[mhdawson]

@ForestEckhardt wondering if you might have time to help review/land?