Lambda layer updating when there were no changes

Why does chalice with `automatic_layer` have to update the lambda layers every time you call deploy? If it knows to use the same deployment package, shouldn't it know it doesn't need to update the layer? This method doesn't really save time like it says in the documentation, because the dependencies are still getting re-deployed.

Example where I run `deploy` twice:

```
$ chalice deploy
Creating shared layer deployment package.
Creating app deployment package.
Creating lambda layer: webstack-dev-managed-layer
...
$ chalice deploy
Creating shared layer deployment package.
Reusing existing shared layer deployment package.
Creating app deployment package.
Reusing existing app deployment package.
Updating lambda layer: webstack-dev-managed-layer
...
```

Start at the deploy entry point for automatic_layer and reproduce the two consecutive deploys shown in the issue. Trace the shared package reuse versus layer update path; done means an unchanged second deploy reuses the existing Lambda layer without updating it, while changed dependencies still trigger an update.

Written by the indexing model from the issue text.