athenaHLT: remove usage of python include when running from DB/JSON

When running in athenaHLT from the DB or JSON files, we use TrigPSCPythonDbSetup.py to setup a minimal Python environment. Change this file from being an "include file" to a Python module.

Introduce helpers functions to alternatively include or import the file based on its file name extension. This will be important to support CA-based configurations.

Relates to ATR-26800.