Add BuildpackForKey helper method.

[robdimsdale]

Summary

This PR adds a helper method on occam.Image to easily facilitate looking up the metadata for a buildpack by its key. This enables consumers to avoid iterating through the buildpack array and/or hard-coding buildpack index.

Use Cases

There are multiple situations (e.g. this, and this) where we hard-code the buildpack index. This is brittle, as it breaks if a buildpack is added to or removed from the build order.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
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• 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).