Default script file behavior

_From @asavaritayal on June 28, 2018 18:42_

If `scriptFile` tag is not provided in `function.json`, the runtime is unable to index the Python functions.

**Observed Behvaior**

If `"scriptFile": "{filename}.py"` is present in `function.json` , the worker uses `{filename}.py` as the script file.

Elif `run.py` is present, the worker defaults to it as the script file.

Else 404 : No job functions are found

**Expected Behavior**

If `"scriptFile": "{filename}.py"` is present in `function.json`, the worker should use `{filename}.py` as the script file.

Elif `__init__.py` is present, the worker defaults to it as the script file.

Elif a single `.py` module is present, use it as the script file. [Nice to have]

Else throw 404 : No job functions are found

_Copied from original issue: Azure/azure-functions-python-worker#159_