Dependency caching on deployment

Currently for Python function apps, the dependencies are built each time on publish regardless of whether there were any changes to the `requirements.txt` file, resulting in extended publishing time.

It'll be great if we can capture a hash of the requirements file and build only if there has been a change.

/cc @brettcannon

Start by tracing the publish path for Python function apps and how requirements.txt dependencies are currently built. Confirm how a requirements-file hash can be retained between publishes; done means unchanged dependencies skip rebuilding while a changed requirements.txt triggers a build.

Written by the indexing model from the issue text.