Refactor publishing and language specific flows

As our code base and hosting options grew, minute additions to the core-tools have made some of our code a bit difficult to follow.
While it's not a mess yet, it'd still be good to spend a little time to think, refactor and clean up some design.

`PythonHelpers` and `PublishFunctionAppAction` would be top candidates for this.

Read PythonHelpers and PublishFunctionAppAction first, then trace how publishing and language-specific flows pass through them. Map their current responsibilities and the hosting-related paths they affect. Done means the design is cleaner and easier to follow without changing publishing behavior.

Written by the indexing model from the issue text.