Fixes caching by accounting for dep_date at metadata root level

[pivotal-david-osullivan]

Summary

Some buildpacks store layer metadata in a dependency field, others do not. Previously, the fix for deprecation_date handling/comparison only accounted for the first scenario. This PR handles the date being at the root level of the layer metadata map.

Use Cases

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).