Adding extension flag in jam pack as a fallback

[pacostas]

Summary

This PR supports using the --extension flag when calling jam pack . At the moment the --extension option is not supported by jam although there is a draft PR for supporting it here: paketo-buildpacks/jam#234

Use Cases

In case of buildpack.toml file is not available, instead of using --buildpack flag to package a buildpack, it fallbacks to using --extension flag in order to package an extension.

Checklist

• 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)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: pacostas / name: Costas Papastathis (50f2345, 1f4e600, 9067e34)
• ✅ login: ForestEckhardt / name: Forest Eckhardt (fcd2a8e)

[sophiewigmore]

I think we should definitely wait on this PR until paketo-buildpacks/jam#234 lands

[mhdawson]

Successfully used this in an integration test in https://github.com/nodeshift/ubi-nodejs-extension/pull/35

[robdimsdale]

In principle I think this is fine, though I have a couple of minor changes I'd like to see. Once we've updated them I'd be happy to merge.

In general, I don't love the idea of silently selecting buildpack.toml vs extension.toml or silently falling back from one to the other, but I'm having a hard time coming up with a better interface at this time. So I'm happy with this general direction.

[robdimsdale]

@pacostas looks good. Can you fix the lint error? (Comparing fileExists == true can be shortened to just fileExists). Then this PR will be good to once paketo-buildpacks/jam#234 is merged.

Marking as blocked on the upstream jam functionality.

[ForestEckhardt]

https://github.com/paketo-buildpacks/jam/releases/tag/v2.3.0