Give packager a wheels directory instead of vendoring wheel contents

It would be nice if instead of unpacking my wheels into the `vendor` directory, I had the option to point the packager to a directory with wheels, so it ingests them in `_download_dependencies()` before trying any other methods.

https://github.com/aws/chalice/blob/1cdebd90d66435c26958687a1d6a3670eeb06293/chalice/deploy/packager.py#L344-L411

Start in chalice/deploy/packager.py at _download_dependencies(), especially the linked lines, and trace how the existing vendor directory and other dependency sources are handled. Define where the wheels-directory option enters the packager and verify that wheels there are ingested before other methods; done means coverage for that precedence and the existing paths remaining intact.

Written by the indexing model from the issue text.