Suggestion: key for rebuilding package on "deploy"

Often on the deploy chalice uses existing package:
```
Creating deployment package.
Reusing existing deployment package.
Updating lambda function: ....
```

What are the conditions on which the package is reused?
I presume that it checks if the `requirements.txt` is changed?

It is possible to have local package in the `requirements.txt`, which is installed internally in editable mode (`pip install -e .`). In this case when the code of the library is changed, chalice will still reuse existing package on deployment. Same would happen if `requirements.txt` points to a branch in git repo.

It would be great if there was an option to force-rebuild package on deploy to cope with such cases.

The issue does not name implementation files or tests. Start by locating the deploy path that emits “Reusing existing deployment package” and inspect how package reuse is decided; done means providing a way to force a rebuild and covering the behavior with tests.

Written by the indexing model from the issue text.